Honestly if you are going to come out of the closet, there is no "proper" time to do it. There never will be, never has been. But the holidays are just the ABSOLUTE WORST time. My mother found out I was "pansexual" and found out what it meant when I was 15.
She didnt know much english (we are polish), and this was around christmas. And I came home from school, she had my diary in hand. The first words out of her mouth in english "YOU RUINED CHRISTMAS!!!!!!!!!" She called up my grandparents earlier that day saying "We are going to church, and that we wont be attending christmas." I mean I had learned that christmas "KEEP A LOCK ON YOUR DIARY!" But when we went to church I ended up covering my butt and said "I have seen my sinful ways" And they are convinced I am straight now. The whole family is.
SO! The best way to do it is just to lightly bring it up in conversation. Try to get them involved in homosexual conversations BUT! Acting as if you are a simple supporter of LBGT people rather than outing that you are one. Then see how they react.
Thats how I found out not to tell my parents later on in life.