"Let not him that eateth despise him that eateth not; and let not him which eateth not judge him that eateth: for God hath received him. Who art thou that judgest another man's servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand.
One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it."
(Romans 14:3-6)
I personally am thankful for my life. I thank God for all he has given me, including my life, and so I celebrate my life on my birthday, as well I celebrate the lives of all my dear loved ones on their birthdays, and I even celebrate the life of my Savior. While I might not know the actual date of Jesus' birth, I do know he was born, and I can celebrate Him coming into this world, Thank God, on any day I choose. If I knew his birthday, I'd celebrate His birth on that day, but honestly, give me a reason to celebrate, and I will celebrate. I welcome every occasion to give thanks to God.