BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
RRULE:FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=SU,MO,TU,WE,TH,FR,SA
SUMMARY:Move $$$ into savings
DTSTART;VALUE=DATE:20181130
SEQUENCE:0
END:VEVENT
END:VCALENDAR

Create file to add event to last day of month
Create file to add event to last day of month

Save this file locally as an .ics file, and then import into a Google calendar. To update, change the summary to be the title of your event, and set line 6 to be the next last-day-of-the-month.