Changing due date??

Way back when I went to my initial OB appointment in the 8th week my OB went by my LMP to determine my due date and that was December 21st but when I had an ultrasound at 8 +4 my due date changed to December 23rd. I had an ultrasound at 12 weeks but I didn't know which due date they were using because my doctors office doesn't change it if it's a couple days different. I have an ultrasound on 7/11 at 16 +3 and is there a chance my due date will change once again?? I'm really trying to stay before Christmas due to certain circumstances!