Volo's Tractate on the Waterdavian Calendar

Waterdeep follows the Standard Faerûnian Calendar, which has twelve months that roughly correspond to the months of the Gregorian calendar. Each month has thirty days divided into three ten-day weeks (each week is called a tenday). Scattered throughout the year are five holidays that aren't considered part of any month. Once every four years, the holiday of Shieldmeet is added to the calendar as a leap day immediately following Midsummer night. Collectively, these days create a cycle similar to what we have on Earth: three 365-day years followed by a leap year. 
The Faerûnian months are Hammer, Alturiak, Ches, Tarsakh, Mirtul, Kythorn, Flamerule, Eleasis, Eleint, Marpenoth, Uktar, and Nightal.
The annual holidays are Midwinter (between Hammer and Alturiak), Greengrass (between Tarsakh and Mirtul), Midsummer (between Flamerule and Eleasis), Highharvestide (between Eleint and Marpenoth), and the Feast of the Moon (between Uktar and Nightal).

Note from Toby

I haven't gotten the annual holidays quite figured out yet in the calendar. The 31st days of Hammer, Tarsakh, Flamerule, Eleint, and Uktar are actually the holidays as explained above.