I hope everyone knows that they should pay their credit card bill on time. However, you might not know why you should.
The most direct reason is that the card issuer will charge a late fee. Typically, this will be $29, but check your agreement for details. This fee is over and above whatever new purchases you make and the associated finance charges. It also gets added to your balance and therefore you get charged interest on it.
Your issuer might up your interest rate for being late. Late payments can take you out of the "in good standing" interest rate. Not surprisingly, the interest rates for customers in good standing are lower than for customers who aren't. You want to be in the good standing category.
After enough late payments, your issuer might put an entry on your credit report. This can affect your interest rates for all of your debts. It makes you look like a bigger risk, so your creditors will charge more interest.
When it comes time to pay your credit card bill, the due date listed on your statement does not mean the check has to be postmarked by that date. It means the check has to get to the issuer by the date. To be safe, you should plan for your check to arrive a day or two before that to allow for processing delays at the issuer. Mailing your bill a week before it is due should be safe, but if you live out in the middle of nowhere, you might want to allow for more.