I've put together some yearly calendars for the years 1220 through to 1300. Mainly so that players know when the next moon phases/equinoxes/solstices are coming up, with regards to casting spells with durations Moon or Year. I thought that, since I already made the PDFs for them, I might as well share them.
I've also included the main religious dates that need calculation, for the three main Divine religions: Judaism, Christianity and Islam. The dates included are Yom Kippur, Easter Sunday, the start and end of Ramadan, the Laylat al-Qadr (the Night of Destiny), and the start and end of the Hajj. It should be noted that the start of months in the Islamic calendar is dependent on physical observation of the first crescent of the given moon, thus any given month can start and/or end differently than shown in different parts of the world, due to the moon rising earlier in the west, and also due to different observations in different places. This means that both Ramadan and the Hajj can be placed with a difference of up to two days in any given case, thus also moving Laylat al-Qadr accodringly. It should also be noted that, even though the night of Laylat al-Qadr is given in Realms of Power: The Infernal as being on the 27th day of Ramadan, it can actually be any odd night of the last 10 days of Ramadan, though the 27th is the most common.
You'll find three ZIP files with two PDF files in each of them. I made A4 and Letter sized calendars, each in a single ZIP file with a full-color and a printer-friendly PDF version. Me being from Spain, the weeks in the calendars start on Monday rather than Sunday. It was rather a lot of work to change that, but I thought you might find them useful anyways. There is also an additional A4-sized calendar in Spanish, also in color and printer-friendly versions.
As for the moon phases and equinoxes/solstices calculations, I made use of AstroPixels, which supposedly calculates them using the Julian calendar instead of the Gregorian calendar for dates prior to 1582. I used the Julian calendar from Time and Date for calculating which weekday corresponds to which day in the month. For Yom Kippur, I've calculated the Julian equivalent of Tishri 10th of each year at the Fourmilab website. I'll talk more extensively regarding Easter Sunday in a while. Finally, I've made use of Fourmilab to calculate the equivalent Julian dates of the Islamic calendar months.
The calculation of Easter Sunday after the First Council of Nicea (AD 325) places it in the first Sunday after the first full moon that happens on or after the spring equinox. But the Church used March 21 (the Ecclasiastical Equinox) as the date for the spring equinox, instead of the actual date of the equinox. More so, the date of full moon uses a calculated date (Paschal Full Moon) instead of the actual date of the full moon. The Paschal Full Moon, which nowadays is calculated through complex formulae, was calculated in the 13th Century from tables representing the 19 year Metonic Cicle. Easter Sundays in these calendars use the Paschal Full Moon calculation according to the Metonic Cycle and using Julian dates, which means they are different from what sould be calculated today, but use instead the system in use at the time.
Obviously, there's no guarantee whatsoever that those two resources are right for medieval dates, particularly regarding Fourmilab. But this is only a game, not serious research, so I guess it's probably ok.
Hope you find it helpful!
Edit 1: Updated the link since I extended the calendars up to AD 1240. Updated the description as well. (2016-04-15)
Edit 2: Updated the link since I upgraded the calendars to include some significant religious dates. (2016-04-18)
Edit 3: Updated the link since I extended the calendars up to AD 1300, and used a system for calculating Easter Sunday which is more appropiate for the 13th Century. Updated the description accordingly. (2020-03-24)