A celebration every consecutive month that you have been in a relationship with your boyfriend/girlfriend.
An excuse to buy her a present.
Something that she might bitch at you for forgetting.
A good day to pig out on cake and/or make out.
If you're in high school, a motive to skip homework and go play lazer tag.
Another walk of life celebrated by a useless holiday.
A contest to see how many months more you can stand one another than another couple.
Just another way to express undying love, or some similar feeling of mindless infatuation.
"Ted, where the HELL IS MY MONTHIVERSARY PRESENT, you cheap bastard??"