I'm creating a spreadsheet to see where I am with my work each month. I am a vendor for car dealerships and input each invoice into the spreadsheet. for example; column a is the date, column b is my invoice number, and column c is the price. on any given day I may have anywhere from 2-20 invoices. how do I put the daily average into a cell?

It seems you're putting your data in a form perfect for pivot tables; give us an example spreadsheet to demonstrate possible solutions.