Please reserve your seat(s) by 18:00, 8 days before the performance.
Make sure you have your ticket(s) booked by 6p.m. at least 8 days prior to the performance.
ーPlease reserve your seat(s) by 18:00, 8 days before the performance.
「公演8日前の18:00までに座席をご予約ください。」
何を reserve するのかまで言うともっと自然な言い方になりますよ。reserve なら seat(s) が良いでしょう。
by 18:00, 8 days before the performance で「公演8日前の18時までに」
ーMake sure you have your ticket(s) booked by 6p.m. at least 8 days prior to the performance.
「公演8日前の午後6時までに必ずチケットを予約してください。」
by 6p.m. at least 8 days prior to the performance で「少なくとも公演8日前の午後6時までに」
ご参考まで!