Recently, I've really become a big proponent of the "Keep it Simple, Stupid" mentality of game/setting design. I firmly believe keeping things simple and concise makes said things a hell of a lot easier to use at the table.
This is why I decided to utilize a simple base for this calendar, one that's incredibly easy to grok. This calendar, which I'm calling the Rhestiloran Calendar after the kingdom that once ruled the Elsir Vale, uses the four seasons as its base. Each season is further broken down into three months each, that have 30 days divided between them. The following are the four seasons and the common names for each month.
SPRING: Newyear, Rebirth, & Storms
SUMMER: Light, Sowing, & Growth
AUTUMN: Toil, Harvest, & Rest
WINTER: Snow, Passing, & Yearsend
Originally, I have the seasons divided into 8 months. However, I decided 12 months would be better due to it mirroring the traditional Gregorian calendar, meaning players had a familiar concept to latch onto. Furthermore, 12 months allowed me to have 30 days instead of 45, making it easier to divide into 6 weeks. This means I'll have to sacrifice the familiar 7 day week for a 5 day one, but I'm okay with that.
I also wanted to keep the names simple so they were easier to remember. I could have given them exotic, fantasy names, but I can guarantee you I'd forget them and would kick myself in the ass for doing so. I also feel like these simple names give a quick description of the month's place within the year. For example, figuring out what activities happen during the Month of Harvest shouldn't be too hard. I'll be using a simple naming scheme for the days of the week as well.
This is just one method. I could have also gone with a lunar-based calendar, having 13 months derived from the phases of the month, or just created a fantasy version of the real world calendar system by just replacing the names. However, I wanted to try something different and I think it turned out pretty well..