onsuchcolddays用介词on而不用in,在这里是不是指“具体的”这些冷日子那intheolddays怎么不用on呢

Question

on such cold days用介词on而不用in,在这里是不是指“具体的”这些冷日子
那in the old days怎么不用on呢

Answer 1

供参考：
1.(in / during /over) these/those days
2.in / on the winter days
3.in the old days
4.on / in such cold days
5.on a cold day,on a hot morning / afternoon / night