Only one observance is commanded by Jesus and that is to meet with fellow believers and share bread and wine in remembrance of his sacrifice.
1 Corinthians 11:23 For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25 After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26 For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come. 27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. 28 But let a man examine himself, and so let him eat of that bread, and drink of that cup. 29 For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body.
No day is specified for this, only that it should be done often. The early church found it convenient to meet on the first day of the week (Sunday) and this became a tradition.
Jesus was certainly not born on the 25th of December and no commandment is given to observe his so-called birth-date. Similarly, no commandment is given to celebrate Easter. Christmas and Easter observance are man-made and unnecessary.
The celebration of birthdays is a matter for the individual as there is no religious connection.
I hope you find this helpful.