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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