I have gathered up some historical information to answer this mystery question, Since in use are two form of calendars. 1) Lunar Calendar 2) Solar Calendar.
Lunar Calendar follows cycle so can only have either 29 days in a month or 30 days in a month, no more than 30 days and no less than 29 days though have twelve months like in Solar Calendar but for Solar Calendar, each month having same amount of days unlike in Lunar Calendar for instance take the 1st month of a year may be of 30 days next year, same month may not be of 30 days. Same goes for all twelve months.
Gregorian Solar Calendar only having their 2nd month of 28 days maximum for three years and have to add one more day to make same month of February as of 29 days and this year shall be called a ‘leap year.’
Edit: Actually every month having 28 days or more, certainly both in Lunar and in Solar Calendars.
Every month has 28 days out of 12 months in a year. Most of them are have some extra added days. Let's see how it is.
January → 28 days + 3 extra days
In February there are 2 possibilities:
February → 28 days or 28 days + 1 extra day
March → 28 days + 3 extra days
April → 28 days + 2 extra days
May → 28 days + 3 extra days
June → 28 days + 2 extra day
July → 28 days + 3 extra days
August → 28 days + 3 extra days
September → 28 days + 2 extra days
October → 28 days + 3 extra days
November → 28 days + 2 extra days
December → 28 days + 3 extra days.