Yes, there are some days in a woman’s cycle when pregnancy is more likely and some days when it is less likely. However, it is possible for someone to get pregnant any day of the month, even on their period.
This is possible because sperm can live inside a female body for up to seven days! That means someone having unprotected sex on their period could get pregnant if the sperm is still hanging around after their period ends. It is also possible for more than one egg to be released from the ovaries during someone’s cycle. That means if someone is on their period, a second egg could be released and there could be a chance of pregnancy (see internal female anatomy). And finally, it’s common for younger folks to have unpredictable periods from month to month, so there are no 100% “safe” days to have sex without a chance of pregnancy.
Also, people still have to think about STI protection when they are on their period. If people are having sex, then that means using a condom everytime.