The question you need to ask yourself is "why do I need to drink once a month?"
If the word 'need' is in any part of your answer, be alarmed.
I tried the once a month thing. it turned into twice a month, then three times a month, then the "event" lasted multiple days and so on. In no time at all I was drinking every day again.
I now realize this was just my way of rationalizing my desire to comsume alcohol and get drunk. I would give myself an inch and then take a mile, one inch at a time.
I hope it works out for you, but I assume since you are here, theres a good chance you have a drinking problem. People who don't have a problem don't need to give theirselves a limited number of days per month to allow themselves to drink.