Meaning of ON in English
( prep. )
At the time of, conveying some notion of cause or motive; as, on public occasions, the officers appear in full dress or uniform. Hence, in consequence of, or following; as, on the ratification of the treaty, the armies were disbanded.
Webster's Revised Unabridged English Dictionary. Пересмотренный полный словарь английского языка Webster. 2012