"What phase is the moon?"

I have a moon phase die from when I used to play Werewolf: the Apocalypse. Although in truth I'd probably just use a d8 or something rather than digging it up.

I use https://app.fantasy-calendar.com to track not just the phases of the moon, but also major events in the game, days passed, holidays, merchant schedules, etc. Granted that matters a lot more in my game for story reasons.

If I were you I'd roll a D10 to determine the current moon phase and then start tracking it from then onward.

I always track the phases of the moon. I built a program years ago that keeps track of tides, moon phases, sunrise and sunset times, and weather.

Most of those are just basic math and were pretty easy, weather was the tricky one, and it more randomly generates weather than just keeps track of it.

I think it adds a lot of flavour and keeps things fresh when I know, for example, that it's the middle of winter and the party is north enough that the sun sets at 6pm and it's a new moon so it gets dark quick. Or that it's a clear, sunny, but very windy day. It can even have mechanical consequences if they are tracking (or being tracked) and it's raining or there is light from a full moon or whatever.

And by "program" I of course mean an overly complicated excel spreadsheet with some VBA.

It depends.

If I wanted to create a sense of urgency, I'd make it soon. Depending on the situation and the resources the players have at their disposal, it could be days or weeks until the next full moon.

If I wanted to give them plenty of time, I'd say the full moon just ended and they have a whole Lunar cycle.

If I felt like being random, I'd roll a d2 to determine if the moon is waning or waxing and then a d6 to determine the phase: 1 - full, 2 - almost full, 3 - gibbous, 4 - half, 5 - crescent, 6 - new

I would either arbitrarily declare a phase or randomly select one, and from then on start tracking the cycle.

For reference the synodic month is approximately 29 and a half days. (Time between new moons, and theoretically also the time between any phase of the moon and itself)

Fun fact, there are 5 lunar months based on different ways of measuring, and the weirdness of orbital mechanics. Cool for advanced world building, completely unnecessary for a typical campaign where you can just say the time between new moons is perfectly X days cause thats how the gods made the world.

Summarizing Wikipedia:

Synodic: time between new moons (≈29.5 days)

Sidereal: time to orbit 360° and measured against background stars. (≈27.3 days)

Tropical: time between right ascension crossings of the ecliptic (the path of the sun) and is a couple seconds sorter than a sidereal month. (≈27.3days)

Anomalistic: the time between perigees (closest point to earth in its orbit) which also means the time it takes the apparent size of the moon to complete a cycle. (≈27.6days)

Draconic: the time between the moon passing through Earth's orbital plane in the same direction, these points are called ascending and descending nodes. (≈27.2days)

Whatever you feel makes for the best narrative. Give them a tight but possible timeline.

This is one of many reasons I use Fantasy Calendar. I used to track this stuff by hand – literally just marking out the days and moon phases on a paper calendar, like a caveman! – but this site lets me automate nearly everything. Moon phases, weather, holidays, etc.

You definitely just pick based on what seems like the most fun.
I’d err towards a timescale that forces a tough decision on the party - to get cured before the first transformation means a diversion that might mess up some other goal. Maybe it’s best to just try chaining the pc up for a night and hoping they don’t break free, then curing them next month?

But I run variant Lycanthropy anyway so that might affect things

What if there's more than one moon?

Asking for Oerth, Krynn, Eberron, Mystara, Athas ...

I've seen a similar problem when a party member suddenly needed to start knowing exactly how much sunlight was present at any given time.

Roll (10d6)/2, they have that many days before the next full moon.

EDIT: added the forgotten division and changed the number of d6 to match reality slightly better.

I check my fantasy world calendar. I always track the days and the phases of the moon are on it.

I'd just go by what's happening in my area unless it's a full moon.

If it's a full moon, roll a d8 and assign a number to each phase of the moon. This website has a chart front and center. New moon is 1. Full moon is 4. Reroll on a full moon.

It depends on the setting, but here a lunar phase changes about every 3-4 days. So if you roll a 3, they have 5 lunar phases or about 17ish days before they go on a midnight tear.

Edit: I'm dumb.

4 is a full moon since it's a cycle. They would only have 3-4 days in my example.

Depends on the setting. For example, on Eberron, you're shit outta luck

Narrative discretion. I'd say either: a) just long enough to finish the current mission and get back to a temple for a cure [clock is ticking, no time to waste!] or b) long enough to make it to a temple if you abandon the current mission [time for an RP moment, what is more important?]. One of those options is much harsher than the other.

The moon in my world has been shattered, which causes lycanthropy to proc randomly (I just use a d12 rather than worry about tracking).

Tip: you don't need to track the moon's phases accurately with regards to time passing in the game, you just need to track them more accurately than the players do.

We're in Faerun so we are using the offiicial calendar of harptos and I track the dates. There is an official calendar of harptos online that includes the phases of the moon. There are also global events as a bonus.

My party had to kill NPCs they already killed once because they were actually wererats who can't die without silver, which they learned upon their vengeful return on the night of a full moon, both in the game AND in real life!

Roll a d6 and a d10. The d6 will generate the tens digit, 0-2. The d10 generates the ones digit. So if you roll a 3 and a 7, the next full moon is 17 days from now. If your moon only has a 28 day cycle, reroll if you roll 29 or 30.

Why wouldn't you just track the days? There's a ton of reasons outside of werewolves why you would. Use an IRL calendar or FR one if you don't want to make one up.

If you really want to just track one full moon to the next, then just count the next 28 days.

This came up in our PF2E game just this week. Golarion's calendar is aligned to Earth's and we were investigating some wolf-like murders. I suggested a werewolf as a possible murderer and asked about the moon phase, DM just says "google moon phases in July 1929!"

I had to do this while running ToA. It was pretty simple, I just rolled a dice then kept track of the days.

General advice: if you’re unsure about something, roll dice about it! Bonus points if you explain the stakes and roll in front of the board. If the dice say it’s a full moon tomorrow that feels more fair than the DM simply declaring so

Just roll a d29.5

The original Dragonlance book had a system for tracking the three moons, cause each of the three orders of mages had their spellcasting affected by their respective moon's phases.

I recently bit one of the PCs in my DoIP game, and he failed his save by 1. I rolled a d30 (we're playing on a VTT with Discord VC) to determine what day of the lunar month it was. He has about a week before the full moon, so when they go into Neverwinter Wood next session, they'll be looking for wolfsbane as a stay on turning, but it won't cure him.

I used fantasycalendar to set up a simple system

I will simply make it up.

If I want the lycanthropy to be a background plot hook to bring up in a few sessions, they’ve got almost a full month. If I want it to now be the big crisis, they’ve got a few days. There aren’t any rulesets for PC lycanthropy other than “you are now an Npc”, so the process takes however long or is as dramatic as I want for the campaign. I love werewolves, so it’s probably gonna take up the limelight.

Isn't this a simple matter of date tracking?

Here's the moon, month and time tracker from my D&D campaign, adapted to my upcoming board game ...

Tell the player asking to roll history. Below ten, they don't know (but it's definitely going to be a full moon soon). Above twenty, they know and have plenty of time. They might even remember facts about how to deal with lycanthropy or where to find an expert on it. 10-20, chose a value that works well for the story (or roll for it), and let the player know. 

This is why my game has a lunar calendar, also to track the changing months.

I have a lycanthrope NPC planned, and have decided there are 3 moons. This world has variant full moons that can not be tracked. I dont deal with moon phases, but do roll D8+D10+D12 each day to see if a moon is full (on a 1). This is a 10% chance per day. If there is one full the NPC leaves the party to hunt but knows its friends. If 2 full moons are present it must roll a Wisdom save or attack its party mates. If all 3 end up 1s, well you can figure that one out...

When I created my world I wrote down the moon cycles so I’d never have this problem. But if you haven’t done something like this then honestly pick an amount of time is fair.

Make your moon weirder and easier to manage.

Put it in a geocentric orbit so that the phase of the moon is tied to the time of day. Plus, only half the planet even knows about the moon.

Make the moon rise with a phase that's random every day. Anyone who spends too much time questioning how the moon's phase is irrespective of where the sun is ends up disappearing.

One of my players contracted lycanthropy, but he doesn’t want to cure it because he wants the non-fullmoon benefits, so at the start of each adventure we roll a d20 to see how many days left until the full moon (I recommend changing the die size to suit your campaign style).

As a neat side effect, it puts a soft time limit on the adventure which limits long rest abuse.

It's the phase that fits the plot.

A friend of mine used a d30 to determine the number of days until a full moon. The reasoning is, if your D&D world has an earth-like moon, then there's guaranteed to be a full moon every 30ish days.

As with so many things RPG, donjon is your friend.

https://forgottenmaps.web.app/map/Sword%20Coast has a calendar function with moon phases

I track the days by date at the top of my notes with important notes below it stating important events. As for tracking the moon specifically, you can make it really easy on yourself and use the dnd calendar which has exactly 30 days in each month. Just say the full moon happens on X day of every month. For me, there is a full moon every 15th right in the middle.

You don't have to have your campaign on a world with a large, single moon. The Earth is unusual in that regard. Also, even in old D&D, a character who was afflicted with lycanthrope would eventually be able to change at will. The actual lore also indicated that changes were at-will although the full moon made it more likely to result in an unwilling transformation.

I adjust the moon and planet and calendar so the beginning and end of the month is always a new moon and the middle of the month is always a full moon.

In Forgotten realms, all months are precisely 30 days, so I just make moon cycles 10 days long with the full moon on the 10th, 20th, and 30th of each month. It makes tracking super easy to remember.