expression, noun
held unless heavy rain; canceled only in case of heavy rain
A set phrase used in event announcements to mean the event will proceed as scheduled unless there is heavy rain. Often seen on posters, tickets, or websites for outdoor events.
明日の運動会は小雨決行です。
Tomorrow's sports day will be held unless there is heavy rain.
小雨決行なので、少
Since it's 'held unless heavy rain,' the game will take place even if it rains a little.