Kourtney Kardashian and Scott Disick welcomed their third child Sunday; which was already a very special date for the reality star couple.

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Kardashian gave birth to the newborn on her first son, Mason Disick's fifth birthday, Radar Online reported.

They announced on a recent episode of E!'s Kourtney & Khloe Take The Hamptons that Kardashian was expecting a boy. Kardashian and Disick are already parents to Mason and Penelope,2.

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Kardashian and Disick have yet to confirm the news of the baby's arrival, but they Kardashian did take to her Instagram account to wish Mason a happy birthday.

Kardashian shared a snapshot of Mason giving the camera thumbs up with an adorable smirk.

She added the caption, "Happy birthday to the coolest little boy in the entire world. You teach me so much every single day and changed my universe the second I met you. You have taught me what life is all about and I am truly blessed and honored to be your mommy."

Kardashian's sister Kim Kardashian also gave Mason a shoutout for his birthday. She shared a photo of herself and Mason with her nearly 24 million followers via Instagram. She wrote, "Happy Birthday to the lego master Mason!!! You are so big I can't take it! You are my everything!!!!! I love you so much!!!!"

Khloe Kardashian posted a photo of Mason and wrote, "Stop growing up! How are you 5 already?!?! My ace! My Mase!"

The Jenner clan also showed love to Mason on the family's big day. Take a look at more pictures below.