Occasions are quite rare I have the time to spend a day with

Occasions are quite rare I have the time to spend a day with my kids.
Occasions are quite rare( ) I have the time to spend a day with my kids.
A. who B. which C. why D. when
Occasion表示场合的时候是什么时间方面的还是地点方面的,怎么理解
intance的场合的意思怎么理解.
也就是用when还是where