The Last Day of The Month

RRULE:FREQ=MONTHLY;BYMONTHDAY=-1

Similarly, if you want an occurrence on a day before the last day of the month, use BYMONTHDAY=-2.

Note, that if you specify BYMONTHDAY=31 then according to the iCalendar standard, there will be no occurrence generated in the months that have less than 31 day.

The Last Workday of Every Month

RRULE:FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1

The above rule specifies all workdays of a month and selects the last of them. The net result is a last workday in a month.

The Last Monday of The Year

RRULE:FREQ=YEARLY;BYDAY=-1MO