Scripture does not specifically lay out the day that Yshua was born, nor does it command us to celebrate his birthday. Ironically though, Yahveh's people are commanded to celebrate the Feast of Sukkot (Tabernacles), a seven day festival beginning on the 15th day of the 7th month and ending on the 21 day of the seventh month. Though distinct from the 7 days Scripture designates for Sukkot, Hashanah Rabbah, the 8th day of Sukkot, is for all practical purposes part of the celebration of Sukkot. Y'shua was born on the first day of Sukkot and circumcised on the 8th day. For details regarding how Y'shua's birth on this date is calculated from Scripture, click on the "More" button in the header of this site. Then from the dropdown select "the Moedim" and "Sukkot". The calculation is explained toward the end of that article under the heading, "Birth of Y'shua"
So, although the B'rit Hadashah (Gospels and Epistiles) do not instruct us to celebrate the birth of Y'shua, the Torah does, and Yahveh's people were celebrating the days on which Yshua was born and circumcised for over a thousand years before those events occurred, in the fullness of time.
Galatians 4:4-7 (NIV)
4 But when the time had fully come, God sent his Son, born of a woman, born under law, 5 to redeem those under law, that we might receive the full rights of sons. 6 Because you are sons, God sent the Spirit of his Son into our hearts, the Spirit who calls out, "Abba, Father." 7 So you are no longer a slave, but a son; and since you are a son, God has made you also an heir.
Happy Birthday Y'shuah! We give thanks and praise to Yahveh elohenu (the Lord our God) for sending you to redeem us, to give us eternal life, and to pour out your Ruach Haqodesh (Holy Spirit) upon us!