Can you calculate, on which day is our anniversary? # Puzzle

Can you calculate, on which day is our anniversary?

For my anniversary, I decided to surprise my husband since he is a voracious reader, I decided to collect a lot of books for him.

On the first day of the month, I bought one book, on the second, I bought two and on the third, I bought three.

This process went on till the anniversary and on the day, I had 276 books with me to gift him.

Can you calculate, on which day is our anniversary??

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