Christmas is still over 3 months away, but if you want a LEGO Advent Calendar you are going to want to get them now while they are available. The calendars typically sell out every year and I've never seen them on sale prior to December. And, several hiked up prices are already showing up on Amazon.
Each LEGO Advent Calendar includes 24 different buildable surprises hidden behind doors that you can open each day of December as you countdown to Christmas.
Here's where to find them. Note that if you purchase at Target with your RedCard (more here), you will get an extra 5% savings.