Quick'n'dirty locating events containing recurrence rules,
except (most likely) summertime rules:

	grep -rP 'RRULE:FREQ=(?!YEARLY(;UNTIL=19430919T000000Z)?;(BYMONTH=(2|3|4|9|10|11);BYDAY=(-1|1|2|3)SU|BYDAY=(-1|1|2|3)SU;BYMONTH=(2|3|4|9|10|11)))'