To create a last day of month event in Google Calendar, create a new file "myevent.ics" with the contents:
BEGIN:VCALENDAR CALSCALE:GREGORIAN VERSION:2.0 BEGIN:VEVENT RRULE:FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=SU,MO,TU,WE,TH,FR,SA SUMMARY:Title here. DTSTART;VALUE=DATE:20160331 SEQUENCE:0 DESCRIPTION:Description here. END:VEVENT END:VCALENDAR
Update "SUMMARY:Title here.", "DESCRIPTION:Description here.", and "DTSTART;VALUE=DATE:20160331" as needed.
The recurrence rule (RRULE) field with BYSETPOS=-1 causes the event to be the first day of the month less 1 day.
Import into Google Calendar:
Google Calendar warning: "This event has a recurrence rule that cannot be edited in Google Calendar".