Happening once a month - Monthly
Happening twice a year - Biannual
Happening once in two year - Biennial
Happening every three months - Quarterly
Happening at night - Nocturnal
Happening during the day - Diurnal
Happening before something - Prior
Happening after something - Subsequent
Happening at the same time - Concurrent
Happening unexpectedly - Unforeseen
Happening repeately - Recurring
Happening naturally - Spontaneous
Happening everywhere - Ubiquitous
Happening regularly - Routine