December Starts on The Same Day of The Week as September...

Credit: getdp.net

December 1 is the 335th day of the year (336th in leap years) in the Gregorian calendar.

December starts on the same day of the week as September every year and ends on the same day of the week as April every year. In common years, April and July of the previous year start on the same day of the week as December of the current year as a common year and October of the previous year always starts on the same day of the week as December of the current year as a leap year while January of the previous year starts on the same day of the week as December of the current year as a leap year and a year immediately after that. 

July of the previous year ends on the same day of the week as December of the current year as a common year. 

December gets its name from the Latin word decem (meaning ten) because it was originally the tenth month of the year in the Roman calendar, which began in March. The winter days following December were not included as part of any month. 

Later, the months of January and February were created out of the monthless period and added to the beginning of the calendar, but December retained its name.

...Source: Wikipedia.org