We're told Elvira passed away a couple weeks ago, but Marlon waited until her birthday to share the somber news. He posted a touching tribute to his mother Thursday, on what is usually a very special day for them ... they share a birthday, and Marlon says it's hard to celebrate another year knowing his mom is gone forever.

The comedian wrote ... "Losing you shattered me into 1000 pieces. I'm putting my life back together piece by piece. You will always be my glue. Miss you. Today I celebrate for the both of. In the midst of this hurt... i just love you ma."

Marlon adds ... "i accomplished so much, made you so proud... but now i got angel to lift me. #loveofmylife #bdaygotl s*** ma, i gave you all my bdays... wth do i do now 😂 millions of weetahs sugtas and homies #myfirstmylastmyeverything rest well."

Damien Wayans also commented on his uncle's post ... "1,000,000 weetahs Indeed!"

Let's face it, Hollywood owes Elvira a debt of gratitude ... the woman brought 10 Wayans children into the world, giving birth to one of the biggest and most successful families in show biz.

Elvira's kids -- Keenan, Damon, Shawn, Marlon, Dwayne, Kim, Nadia, Elvira, Diedre and Vonnie -- have written, directed, produced and starred in countless movies and TV series, including "The Wayans Bros.," often working together on their content.

Elvira was 81.