Definition 1 of 1
For a people who would rather say "Increase the volume" than "Turn it up", it's not a stretch to imagine why "Many Happy Returns of the Day" is the preferred stand-in for "Happy Birthday."
Going hand in hand with the cultural norm of warmth, politeness, and well-wishing that brings you greetings such as "Have you eaten?" the phrase "Many happy returns..." accomplishes the task of acknowledging the special occasion while blessing you with many more to come.
Fell swoop sweetness is what it is. Welcome to our subcontinent.
Sumant: "Cool cool. No problem. Bugger! Many Happy Returns of the Day!"
Jagan: "Umm... dude... I have 3 months to live..."
Sumant: "Oh...... Then? How are things otherwise?"