"Happy Monday" in Japan is a system to move some of the holidays to Monday, so that people can have a three-day weekend.
この場合、次のようにハッピーマンデーを説明できますよ。
ー"Happy Monday" in Japan is a system to move some of the holidays to Monday, so that people can have a three-day weekend.
「日本のハッピーマンデーとは、祝日のいく日かを月曜に移動させることで、人々が3連休を過ごすことができる制度です」
to have a three-day weekend「3連休を過ごす」
ご参考まで!