I didn't realize the department store was having a big sale.
I was late getting there, so all the big sale items were gone.
この場合、次のような言い方ができますよ。
ーI didn't realize the department store was having a big sale.
「デパートで大きなセールをやってることに気が付かなかった」
ーI was late getting there, so all the big sale items were gone.
「行くのが遅くなって、大きなセールアイテムは売り切れていた」
ご参考まで!