it was on the day when we had our english evening last week that we first me
when we had our english evening last week 是定语从句 修饰先行词day
这是个强调句型：
it was （on the day when we had our english evening last week ）that （we first me）
就是 ……我们……
用when是因为we had our english evening last week本身是主谓宾结构 句子成分完整 只缺少时间状语 所以用when