If you have sex a day before your period stops, what is the possible chance to get pregant?

Unlikely: If you have sex during your bleeding, it is highly unlikely to get pregnant. You ovulate 14 days before your period, and that is the time you are most likely to get pregnant. Which generally is "mid-cycle" if you are regular.

Low but not zero: Timing is not a reliable way to determine when you might get pregnant. It may in fact, be that you are already pregnant since some women have "menses" several months into pregnancy. . Over the counter pregnancy tests are quite reliable. But do take some time. Do you want to get pregnant? If you do not, then use protection to be sure.