The date goes in the top right corner of the check, on the short line just above or beside the "Pay to the order of" line. Write the full date — month, day, and year.
Can you postdate a check?
You can write a future date on a check, but it doesn't reliably stop a bank from cashing it earlier. Under the Uniform Commercial Code, most banks aren't required to check the date before processing a check through automated systems, so a postdated check can clear before the date you wrote. Don't rely on postdating to control when money leaves your account.
If timing genuinely matters, it's safer to hold onto the check and hand or mail it closer to when you want it deposited, rather than postdating it.