History of present day Gregorian Calendar

Roman Calendar (700 BC):

• This calendar was formulated around 2700 years ago.
• It contained 304 days and 10 months
• 60 days were not number since those were winter days
• During winter there would not be any economic or agricultural activities so those were not names
• During that period, Roman consul in Roman republic will decide the number of days in year. They eventually abused their power by lengthening years controlled by their political allies and shortening the years in their rivals' terms of office

Julian Calendar (45 BC):

• This is the closest cousin of present day International Standard Calendar (Gregorian Calendar)
• Seeing the non standard nature of the then present Roman calendar, Julius Caesar order for a massive reform to standardize calendar not to be dependent on anyone
• This reform resulted in introduction of 2 additional months and that was placed in at the beginning of the year 45 BC
• In order to get this new calendar up and running from 45 BC and to follow the solar calendar the year 46 BC had 445 days
• after 45 BC during the entire lifetime of Julius Ceasar this calendar was followed. It had 365.235 days including the leap days 
• In memory of Julius Ceasar, the month of "Quintilitis" was renamed as "July".
• When Augustus Ceaser came to power, he made minor modifications to the calendar without affecting the calendar cycle
• Renamed the month of "Sextilities" to "August"
• borrowed one day from Februarius and increased the number of days in August to 31 and subsequently changing the number of days in following months

Current day Gregorian Calendar (1582 AD):

• This calendar was initially started as a mere correction of Julian Calendar. 
• As mentioned above, Julian calendar had 365.235 days in an year.
• That was not accurate, the number of days in an year should be 365.2425. So 0.0075 days have been added from the date of introduction of Julian Calendar.
• This resulted in jump ahead of approximately 10 days (approx 1500 years * 0.0075)
• When this calendar was brought into practice on 4 October 1582, we had to skip these 10 days to adjust the impact of extension of 0.0075 days per year.
• As a result of this 4-October-1582 was followed by 15-October-1582
• This critical change has resulted in the birth of new version of Julian Calendar and named as "Gregorian Calendar". Named after "Pope Gregory XIII"
• Steps were taken to ensure that the change in calendar does not affect the year sequence (in AD Calendar era), Good friday and Easter.
• This calendar was later considered as International Standard Calendar for the purpose of trade and commerce.