If you carry a balance on a credit card, the bank figures your interest on what you owe each day. It takes your annual percentage rate, or APR, divides it by 365 to get a daily rate, and applies that rate to your balance every day of the billing cycle. The month's interest works out to your average daily balance times the daily rate times the number of days. That means when you pay matters, along with how much.
Start with a $3,000 balance at 24% APR in a 30-day cycle, and assume you add no new purchases. If you wait until the end of the cycle, you owe about $59.18 in interest. Now send $1,500 on day 15 instead. For 15 days you owe $3,000, and for 15 days you owe $1,500, so your average daily balance is $2,250. The interest comes to about $44.38. Sending the same money two weeks earlier saves $14.79.
The savings grow with how early the payment lands. Send the same $1,500 on day 5 and the average daily balance falls to $1,750. Interest drops to about $34.52, which saves $24.66 against waiting until the end.
New purchases change the math, because each one raises the balance from its date forward. Cards also differ in how they figure interest, and your statement will say whether yours uses the average daily balance. Look for that phrase and for your daily rate.
This only matters if you carry a balance. Pay the full statement balance by the due date and most cards charge no interest on purchases, so timing doesn't change anything. Carrying a balance at 24% is expensive, and a $14.79 savings doesn't change that. A home equity line of credit charges interest the same daily way, so the same rule applies there: a payment that lands sooner costs less.
Check your own APR on your latest statement, divide it by 365, and multiply by your balance to see what one day costs you.