When I was planning Senta and the Steel Dragon, I made a very complicated calendar system.  I felt I needed it because I had created a world that was much larger than Earth.  In the end, I threw most of that out.  I was afraid I would forget how many minutes in a day and how many days in a week etc.  I did keep a longer year, though I simplified it.

In the world of The Steel Dragon, they have 12 months of 30 days and a month of 15 days.  I named some of the months for the number of the month (just as they are in our world) and a few for the mythology of my world.  The 30 day months are as follows: Restuary, Festuary, Treuary, Quaduary, Pentuary, Sexuary, Septuary, Octuary, Novuary, Decius, Magnius, and Kafirius.  Hamonth is the 15 day month (half-month).  Because of the uniformity of months and years, I could assign the same dates as the starts of the seasons.

Treuary 21st is the first day of spring.  Sexuary 24th is the first day of summer.  Novuary 28th is the first day of fall.  And Hamonth 2nd is the first day of winter.

I haven’t defined many holidays.  Karfira Mass is Kafirius 25th.  Accord Day is Septuary 30th.