The MonthMask allows you to get or set the month selection, without selecting individual months using GetMonth and SetMonth. The MonthMask is a bit mask, with bits 1-12 representing months 1-12.
For example, to select February and March you would set bits 2 (0x4 or binary 100) and 3 (0x8 or binary 1000), so the MonthMask would be set to 0xC (binary 1100).
To select all months, set the MonthMask to ArcanaDevelopment.adTempus.Shared.SpecialMaskValues.AllMonths.