The mortal calendar follows the planes as they wax and wane relative to the material world. A year has 352 days. The four seasons correlate to the elemental planes, which are dominant for 88 days each.
There are 16 months of 21 days (3 weeks) in each year, four to a season. Each month is named for the Outer Plane that is dominant during it. After each month, the Outlands are dominant for a day.