The statement date is when the issuer closes your billing cycle and summarizes your purchases, payments, fees, and total amount due. Purchases after that date usually fall into the next statement cycle.
The due date is the deadline for payment. To avoid interest on normal purchases, pay the statement balance in full by the due date. Paying after the due date can trigger late fees and finance charges depending on the issuer terms.
For planning, put both dates on your calendar. The safest habit is to pay a few days before the due date and to avoid large purchases right before the cycle closes unless you understand when they will appear on your statement.
Next step: Use the calculators and card database to test the idea against your own spending pattern.