This is a really simple calendar with a perfect 3 month cycle. It is 364 days, with 12 months, and 4 feast days.
Here's a link to the template I use; feel free to make a copy. From there, just copy the template months in order as your campaign progresses.
If you want some more detail for the calendar,
use Donjon. Just paste this code into the "Restore" field, and you've got a set of custom months.
{"year_len":364,"events":1,"n_months":12,"months":["Huginn","Muninn","Valtun","Freir","Plenus","Iuv","Behem","Lavoeth","Plerique","Ederque","Nullusque","Harbardr"],"month_len":{"Huginn":30,"Muninn":30,"Valtun":31,"Freir":30,"Plenus":30,"Iuv":31,"Behem":30,"Lavoeth":30,"Plerique":31,"Ederque":30,"Nullusque":30,"Harbardr":31},"week_len":7,"weekdays":["Solus","Lunas","Marus","Mercurus","Jovus","Venas","Saturnus"],"n_moons":2,"moons":["Luna","Juno"],"lunar_cyc":{"Luna":30,"Juno":28},"lunar_shf":{"Luna":8,"Juno":0},"year":1146,"first_day":0,"notes":{}}