The DayMask allows you to get or set the day selection, without selecting individual days using GetDay and SetDay. The DayMask is a bit mask, with bits 1-31 representing days 1-31.
For example, to select days 1 and 3 you would set bits 1 (=0x2 or binary 10) and 2 (=0x4 or binary 100), so the DayMask would be set to 6 (binary 110).
To select all days, set the DayMask to ArcanaDevelopment.adTempus.Shared.SpecialMaskValues.AllDays.