The 23rd. When a credit card can enjoy the longest interest-free period is mainly determined by the billing date of the current month, and it is most appropriate to start swiping the card the day after the billing date of the current month. For example, the 22nd is the billing date, and the repayment date from 23rd to next month is the longest interest-free period for credit cards. Different banks have different regulations on the specific time from the billing date.