have many more happy days ( refers to occasions e.g birthdays, anniversaries) implying a long and happy life.

Technically, it means, "May you receive many happy things/situations/etc". It is typically used in some places to say "Happy Birthday".

Many Happy Returns is a greeting which is usually reserved for birthdays. It is more common in British English than in American English. The term itself refers to the passing year.

To my mind, it's solely for people's birthdays. "Happy Birthday, many happy returns!". Return to what, I don't know? I never thought about it - I suppose it means...... ummmmmmmmmm...........I don't know......... maybe if someone went away somewhere, to live and work, but they came home every year to see their family for their birthday? I don't know......... many happy, "come back and do this again"s????

See my comment about the Earth, I like that idea :)