represents a day of the week in a month or year such as the
third monday of the month. A negative num indicates it counts from the
end of the month or year.
Each BYDAY value can also be preceded by a positive (+n) or negative
(-n) integer. If present, this indicates the nth occurrence of the
specific day within the MONTHLY or YEARLY RRULE. For example, within
a MONTHLY rule, +1MO (or simply 1MO) represents the first Monday
within the month, whereas -1MO represents the last Monday of the
month. If an integer modifier is not present, it means all days of
this type within the specified frequency. For example, within a
MONTHLY rule, MO represents all Mondays within the month.