simple, ask to have a Christmas Party on the 21 because it's a Saturday. a lot of the Christmas tradition comes from Yule, so no one should be the wiser until you have an actual circle. be sure to either make a Yule Log [decorate a log and carve a hole to place a candle] or buy a Yule Log Cake and place a candle beside it. you're suppose to stay up all night and celebrate, so you could try having your circle when your parents go to bed. my coven has two circles, one at midnight and one at sunrise.
in terms of decorations, again, Christmas, red, green, silver, gold, are common choices, a brightly decorated evergreen, holly, mistletoe. another thing my coven does is a potluck and a gift exchange, so ask each person coming [if you are throwing a party] to bring a little something.
the above link should help shed some more light on the holiday and maybe give you some ideas for things to do or have on that day/night.
on a side note, you should tell your parents at some point you're Pagan. pick the right time, and do it in an adult fashion, but it's best they know sooner rather than later.