The restaurant we were planning to go to was closed for their regular day-off.
We're planning to go to another restaurant owned by the same group/company instead.
この場合、次のような言い方ができますよ。
ーThe restaurant we were planning to go to was closed for their regular day-off.
「私たちが行く予定だった店は定休日で閉まっていた」
regular day-off で「定休日」と言えます。
ーWe're planning to go to another restaurant owned by the same group/company instead.
「私たちは代わりに同じ系列のお店に行く予定です」
restaurant owned by the same group/company で「同じ系列のレストラン」と言えます。
ご参考まで！
カナダ 
