This read-create value's meaning depends on timeRangePeriodicPattern: for daily or weekly patterns it's a 7-bit mask of days of the week (bit 0=Sunday through bit 6=Saturday), while for a monthly pattern it's a specific day-of-month number (1-31). An admin sets this bitmask to select exactly which days a weekly schedule applies to, such as weekdays only. For example, setting timeRangePeriodicDayMask with bits 1 through 5 set (Monday-Friday) under a weekly pattern makes the schedule apply only on business days.