Meaning of ON in English

ON

( 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.