meowsiversary 

The acknowledgement and celebration of having a pet cat for an entire year.

Happy Meowsiversary Guillermo Jellybeans! You have been a member of our fur family for a whole year. Invite your friends over and we'll have cake!

meowsiversary by Friend of Penguin August 5, 2019

mensiversary 

MEN-SI-VER-SA-RY (mèn´se-vûr´se-rê) noun
1. The monthly recurring date of a past event, especially one of
historical, national, or personal importance: a first date
mensiversary; the mensiversary of the founding of Nerstone Pictures.
2. A celebration commemorating such a date.
from Latin: mensis, month + versus, past participle of vertere, to turn.

If the word for the commemoration of a yearly event is anniversary, by
analogy the logical word for the commemoration of a monthly event
should be "mensiversary." "Anniversary" comes from the Latin "annum,"
meaning year. (The "versary" part comes from a Latin word referring
to turning.) The Latin word for month is "mensis." Hence,
mensiversary. I've not found this word in any reputable dictionary,
but perhaps exposure in Google Answers can help to put it there.

Credit to pinkfreud-ga @ google answers

My girlfriend and I just celebrated our first mensiversary.

On November 2nd, the 8th mensiversary of our family reunion in Chicago, my family and I decided to take another trip back to visit the sites.

mensiversary by BeefnCheez November 11, 2007

mensiversary 

Of a significant event, a day that is an exact number of months (to the day, if possible since months have different numbers of days - this conflict obviously doesn't occur with an anniversary) since the event occurred; often preceded by an ordinal number indicating the number of months that have elapsed since the event.
(Keep in mind, this is not a real word, but it could be considered etymologically correct according to Latin and similar derivation to anniversary.)

Could be considered synonymous to lunaversary, a more common fake word for a reoccurring monthly event, but this is less etymologically correct by indirect induction of roots.

WRONG: Happy two-month anniversary!
RIGHT: Happy second mensiversary!

mensiversary by maelstr0m_X1 November 29, 2006

mensiversary 

The monthly recurring date of a past event, especially one of historical, national, or personal importance

Me and my girlfriend are celebrating our first mensiversary next Friday.

mensiversary by TheNad September 4, 2009

Mensiversary 

Term used to express a milestone passed in months, as opposed to years (as expressed by the word anniversary ). Steming from the Latin "mensis" meaning month and the stem -versary denoting a turning of an amount of time.

"Did you hear that Joe and Katy just had their 6 month anniversary?"

"Actually it was there 6th Mensiversary..."

Mensiversary by Happy Mensiversary! July 1, 2009