This is an area as a GM I do not do well. Having a in-game calender I think can add a lot to the atmosphere of a campaign. Not just the weather changers, but holidays, tournaments and other significant events. As a GM this lets me look ahead in time and see that the Midsummer Festival will occur in couple of weeks so I would have the people getting ready and excited. As a GM this is a great time have plotline have some sort of climax during this time or a new on develop.
Calenders can help with developing the plotlines. Plus it helps keep track of time in the campaign. How long it takes to get from here to there. I usually gloss over that part, but this time I plan on keeping one.
To develop my calender i was sorta stuck with names of months and days. What I did is a simple 10 month calender with 4, 10 day weeks. I haven't figured out the holidays yet. For Months and days of the week I google Hungarian words to get inspiration.
Gotta go. Work is busy as heck.