Symbolism, Imagery, Allegory
All girls know that it's no secret that getting your first period is already super-symbolic. It signals that your body is ready to get to baby-making... which is a huge, huge deal. It's a dang milestone.
Some girls are weirded out by this and think of menstruation with the same trepidation they view the milestone of, say, driving your car in a snowstorm for the first time.
Some girls roll their eyes and think of it as yet another chore (to be fair, it totally is) like the milestone of paying your first utility bills. (Woo-hoo.)
And some girls, like Anne, are psyched. For Anne, this is a milestone that's as exciting as a Bat Mitzvah.
At many points in her diary, Anne expresses her desire to grow up so that her family will take her seriously. She hopes desperately to have her period, thinking of it as a physical sign of adulthood. While her period doesn’t make her family respect her maturity, she continues to enjoy it as a "sweet secret."
And like all the other symbols in this novel, Anne's first period is symbolic of just how much devastation was caused by the Holocaust. Anne had just crossed one of the major thresholds into adulthood. She should have enjoyed many more—graduation, a job, a marriage, a family—but her life was taken from her.