In ancient times the first month of the calendar began in March-April and it was been based on the moon phase (as our Hindu Lunar Calendar). Romans could not understand the calendar as it had to have more knowledge on the trails of moon. It was regarded too messy and the Roman calendar was replaced by Julian Calendar in 45 B.C by Julius Caesar with a few changes. When the Julian Calendar which was also based on Lunar phase did not match with the actual time taken by Earth to orbit once around the Sun, which was obvious as the course of Moon and Sun were different it became too clumsy for them.
Fed up with these Moon and Sun phase calculations Pope Gregory XIII in 1582 introduced a calendar called “Christian Calendar” which is now used and it is also called Gregorian Calendar. So the start of year which as somewhere between March-April, now was on January 1st as it was near to the date of Birth of Christ.
This explains why September which means “Seven” (Septa) in Latin is the 9th month and not the seventh month and October, which means “Eight” (Octa) is the 10th month, and not the eighth month. November, which means “Nine,” (Nova) is the 11th month, and December, which means “Ten” (Deca) is the 12th month. At that time, January and February were last two months of the year. April 1 was counted the first day of the year in Europe. When the calendar was changed start from January 1, some people stayed with April 1. Those who did were called “April Fools” and were taunted by their neighboring countries. They called all those who celebrated the new year in April as “Fools”.... And it continues even today......and then they called it April Fools Day......but the Sun and Moon did not stop or change their course. Day never changed, seasons remained the same.
Happy April Fool’s Day……Oops!!!! who are Fool now???……after knowing about it.