With my sandbox campaign I used a broad-stroke at the beginning and have been fleshing things out as we play or as is needed. I've had the cosmology floating in the back of my mind and this is where I'm currently at.
Over at Lamentations of a Flame Princess, James Raggi posted his campaign calendar. I had just so happened to have finished the visual representation of my calendar earlier his week and thought I'd add to the mix.
I love the concept of a lunar calendar and mine is also based on the 28 day, 13 month calendar with one day out of time (Agas). Agas would be a time of High magic and ritual both black and white while the 15th day of the Toad would be a time of only Black Magic potency. The standard 4 seasons apply with the Month of the Phoenix being the beginning of the year and the first day of summer. The moon sister planet of Thuvius (maybe?) waxes and wanes in the 28 days of the month reaching full on the 14th-15th.
I may throw in more Dates of Reverence and Ritual as time progresses but this is what I'll be working with for now.
I liked James' concept of having the players cross off the days as the game progresses. I've been doing that for them and have been giving them an update on the general amount of time that has passed but I'm not sure if they get a real picture. If the players tend to the calendar I think they may have a greater sense of the passage of time and plan events accordingly.