Pins payments to specific weekdays. Only applies to Repeats weekly or monthly. For weekly, provide one weekday abbreviation or a comma-separated list of them — MO, TU, WE, TH, FR, SA, SU (e.g. FR, or MO,WE,FR); each weekday may appear at most once. For monthly, provide exactly one value with a mandatory ordinal prefix, 1–4 or -1 for last (e.g. 1MO, 3FR, -1FR); comma-separated lists are not accepted for monthly. Input is case-insensitive; for weekly, whitespace around each entry is ignored, but a monthly value must have no surrounding whitespace. The stored and echoed value is canonicalized — uppercased and, for weekly, re-ordered into calendar order starting Monday — so ByDay=FR,MO is returned as MO,FR. A weekday repeated within one value is rejected rather than de-duplicated. Weeks run Monday to Sunday: the first payment is the earliest selected weekday on or after StartDateTime, so a schedule starting on a Thursday with ByDay=MO,WE,FR fires only on the Friday of its first week.
