Committing to an offical relationship by mutual choice, and celebrated on the first month after the date that you asked one another to be your partner.
Boy: Will you be my girl friend
(one month later)
Girl: i got you something for our Montaversary.
Boy: Oh yeah. . . Thats so sweet. . . What is it?
Girl: Its a surprise
Boy: I got you something too~
(After exchanging gifts, on your Monthaversary, you are committed to one another.)