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lokesh lokkilokesh lokki 

Create a formula field that displays the value “Yes” if the contact’s birthday falls in the current calendar month

SwethaSwetha (Salesforce Developers) 
HI Lokesh,
Try
IF (
MONTH( Birthdate) = MONTH( TODAY()), "Yes", "")

Reference: https://trailhead.salesforce.com/trailblazer-community/download/file/06930000003yL4IAAU

If this information helps, please mark the answer as best. Thank you
CharuDuttCharuDutt
Hii Lokesh 
Try Below Formula
IF (MONTH( Birthdate) = MONTH( TODAY()), "Yes", "")
Please Mark It As Best Answer If It Helps
Thank You!
The TechieThe Techie
Hi,

Please try-
IF(MONTH(Birthdate) = MONTH(TODAY()), 'Yes', 'No')