I am holding a collection of items in my hands.
I throw two of the items away.

I want to divide it into different groups, but I can only see one half of each item in my hands.

I can easily divide the items into the same amount of groups:

as there are day and night.
as there are seasons.
as there are lunar months.
as there are weeks in the year.

If I add up all the values together,
I get the amount of days in a year (minus 1 for the two items I threw away.)

What do I have?