Checks are "promissory notes," or a promise to pay, drawn on a bank on behalf of an individual or business. Checks feature an "effective date," or a day when the bank will pay the check.
Significance
A check dated tomorrow or any future date is known as a postdated check. A business or individual will write such a check with the intent of paying the check on, not before, that specific day.
Function
Depositing a postdated check a day early may cause the check writer's bank to attempt to pay the check immediately. If the check writer does not yet have the funds in his bank account, this will cause the check to "bounce," or be returned for nonsufficient funds.
Considerations
Checks usually take several days to "clear," or withdraw the money from the check writer's account. This means a check dated tomorrow will often not clear today.