Storing previous result....

Hello,

This is probably ver simple but I cant figure it out. I have a sales spreadsheet that has Month Target and then lists each date in a month in column A, sales made that day in column B, the month to date target in column c (i.e month target divided by number of days elapsed so far in the month), and % against month to date and overall targets in columns d and e. So everyday I enter sales made that day in column b and the rest calculates itself. However what I would like to happen is any given days results for month to date percentage achieved at that time stays the same for that partiuclar day, what is happening is as I go forward through the month and enter each days sales it also changes all the previous days to the same as the current days results. Sorry if I've not explained this correctly, basically once a day has gone i would like to lock down the results data for that day so say on the 20th day of a month i can look back at the 5th day of the month and say 'on the 5th i was x% of target, now im y% of target' anybody help me?