Do call the person on their birthday. Do not wait until Christmas and say 'Happy Birthday and Merry Christmas! We thought we'd save a dime and combine the calls!' Same goes for bdays that fall after Christmas. 2. separate. calls.
That being said, do not combine bday and xmas gifts into the same gift. Your bday's in June, do you get combined bday-xmas gifts?? Yeah, didn't think so.
If your bday person is still under 3 feet tall and to your thinking is 'too young to remember' then think again. I so wish now that I'd been clever enough to formulate the words effectively back then so I could have yelled 'SCREW YOU GUYS I'M OPENING MY DAMN PRESENTS!' Mom, Dad, I love you and while I admit freely to having a crappy memory I have to say this, this I remember... Mandating that 6 year-olds can only open presents during the commercial breaks of football games is total bullshit and could potentially lead to years of therapy. Playoffs or no playoffs. Save yourselves the time, money and guilt and have the parties a) on a day without football, 2) at a venue away from football or c) RECORD THE DAMN GAME AND WATCH IT LATER! This is, after all, the age of DVR, TEVO, and other gadgety stuff.
Do make the events separate. Some how. Some way. The few days before Christmas are often crunch time for everyone, birthday individual included. I can count on one hand the number of times I actually celebrated my birthday on my birthday which now come to think of it is actually ok with me. Believe it or not I'm not even a big bday celebrator. Product of my upbringing perhaps? Dunno. That too could take a separate posting. Between you and me, too much attention makes me uncomfortable. Shocking but true.
For anyone whose bday is actually on the 25th, you're on your own and good luck. I've heard that some people do half bdays in the summer. Hmmm...