"Last summer's (some) day" 的英文表达是 "a certain day of last summer"。

其中 "certain" 意为 "特定的，明确的"，而 "summer" 指夏天，"last" 是"上一个"的意思。

例句:

I remember the day when we went to the beach, it was a certain day of last summer.
He always remember the trip they took on a certain day of last summer
I can't recall the exact date, but it was a certain day of last summer when we first met.

翻译:

我记得我们去海滩的那天，那是去年夏天的某一天。
他总是记得他们去年夏天某一天的旅行
我不能回忆起那天的具体日期，但那是我们第一次见面的去年夏天的某一天。