Because  some scientist  saa that  february months is unlucky month.   soo they  kept that ...
bt why dn't they kept february as 30 daysor 31
The Brainliest Answer!
Actually , some greek or roman civilisation ppl considered march to be the first month of the year and february to be the last . So after distributing 30 and 31 days to the alternative months , they were left with only 28 days for the last month and hence February has 28 days.

NB:It's just a belief.There is no evidence or anything as such
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