Is it safe to cum inside during her 1st day of period?

In what regards ?: Pregnancy ? Use appropriate contraception to avoid unwanted pregnancy. The scenario you describe will usually not result in fertilization, and although outside your partner's fertile period, I never say never. Be smart or be a daddy.

Safe for what?: Reproductive systems of 21 year olds have been honed by evolution over a billion years for ONE purpose; to make a baby. Relying on menstrual cycles, phases of the moon, or physical extraction prior to climax, are all a fool's folly. ANY time you engage in unprotected sex you must assume both, that she WILL get pregnant, and that you WILL transmit AND contract ALL STDs either brings to the party.