Dec is the abbreviation of December. Remember to capitalize the first letter.
By the way, I'd better list the words from January to December for you, hehe. Look at this:
January: January, abbreviated as Jan.
February: February, abbreviated as Feb.
March: March, abbreviated as: Mar.
April: April, abbreviated as April.
May: May, abbreviated as: may
June: June, abbreviated as: Jun.
July: July, referred to as July.
August: August, abbreviated as August.
September: September, abbreviated as Sep.
October: Octorber, abbreviated as oct.
November: November, abbreviated as November.
December: December. The abbreviation is: dec.
I wish you progress in your study! Ha ha.