I wish you would tell me ahead of time when you don't need supper.
I would appreciate it if you could let me know when you're going to eat out.
この場合、次のような言い方ができますよ。
ーI wish you would tell me ahead of time when you don't need supper.
「夕飯がいらないときは前もって教えてくれたらいいのに」
ahead of time で「前もって」
ーI would appreciate it if you could let me know when you're going to eat out.
「外食するなら教えてくれたらいいのに」
to eat out で「外食する」
ご参考まで!