The design of the room was in the fancy style popular in those days为什么是 in those days不是on

The design of the room was in the fancy style popular in those days为什么是 in those days不是on
The design of the room was in the fancy style popular in those days
为什么是 in those days不是on

The design of the room was in the fancy style popular in those days为什么是 in those days不是on
具体的日子用on,泛指一段时间、时期则要用in.