Q: how do i find out when i purchased my macbook?

Other than requesting a copy of the receipt from whoever you purchased the MacBook, there's no way to find out when you purchased it, per se. But you can look up the term of the warranty here:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

and just extrapolate back one year, or three years if you have AppleCare coverage on the MacBook.

Regards.

Or another method, based on the presumption that you have not reformatted your HD since you bought it, is to look at the creation date of it from Get Info.

Creation date on the hard drive would in no way indicate the purchase date of the MacBook unless the OP reformatted the drive on the very day he/she purchased the MacBook. Otherwise that creation date could predate the date of purchase by days, weeks or even months.

Regards,