*The use of a preposition in an adjectival clause that modifies a noun of time is somewhat from that in other adjectival clauses: a preposition is used preceding which. Otherwise, the omitted. Example: A) I'll never forget the day. B)I met you on that day.
I'll never forget the day when I met you.
I'll never forget the day on which I met you.
I'll never forget the day that I met you.
I'll never forget the day I met you.
"Why" We often use why after reason'
Example: A) I can't think of a reason. B) I should help you for some reasons I can't think of a reason why I should help you.