departure date; day of departure
Refers to the scheduled date on which a journey, flight, or trip begins.
The departure date is next Monday.
The departure date is written on the plane ticket.
到着日 means 'arrival date', the counterpart to 出発日.
Compound of 出発 (departure) + 日 (day). The reading しゅっぱつび follows standard rendaku voicing of ひ to び.