Wht is the best way to ask for forgiveness?

What is a monthsary? Do you mean that you forgot your anniversary? If that is the case, why not just spend a couple of days apologising, tell your partner that you've been really busy and lost track of the days. Flowers, chocolates, jewellery will all, I imagine, help to get you back into the good books. Also, make an extra special effort next year, to show that it won't become a thing of habit.