Retaining a value that was based on a date that changes

We're trying to capture the month's payments at the end of each month. See attached file please.

A3 and B3 sum payments each day. When the date reaches the end of the month, we want to calculate the collections for that month ... and record them.

So on 2/28/14 we want values recorded in A5 & B5.

The question (or problem) that we have in doing this: will the value that recorded in A3 and B3 remain in these cells once the date progresses past 2/28/14??

If not, is there a way to "freeze" a value in a cell? Any help would be appreciated!

Thanks!

If anyone has a better way to do this, we'd like to see it too!