例句:let's make it seven. 如果seven变成Sunday前加介词on吗

let's make it sth
这类句子的结构 sth 通常不加介词,
这是习惯,
但在有些情况下要加冠词,
请看例句：
When shall we meet for dinner?
Let's make it 7.
Shall I bring my wfe?
Great idea.I do the same.We'll make it a foursome.