Total numbers of elementary events are: 7

Let E be the event of having 53 Sundays

Note- a non-leap year has 365 days or 52 weeks and 1 day. This one day could be any day amongst Sunday, Monday, Tuesday, Wednesday, Thursday, Friday or Saturday. Out of these favorable event is one; Sunday.

Number of favorable outcome is = 1

P (Sunday) = P (E) = 1/7