It behooveth man, upon reaching the age of nineteen, to render thanksgiving for the day of his conception as an embryo. For had the embryo not existed, how could he have reached his present state?

The Báb: Selections from the Writings of the Báb, p. 89

It is interesting to note that the early believers in the East during the days of Bahá’u’lláh and ‘Abdu’l-Bahá never celebrated their own birthdays. They considered that such an act would imply self-glorification, a means of boosting one's ego. It never crossed their minds that a certain day was their birthday. So real and genuine was this attitude that a great many individuals did not know the exact date of their birth. In the absence of birth certificates, some parents would record the date of the birth of their children in a certain book, much as Westerners used the family Bible for the same purpose. Even then an individual would be deeply insulted if someone wanted to celebrate his birthday, for the only persons whose birthday merited celebration were the Prophets and Chosen Ones of God. Instead of celebrating birthdays, however, these people held regular annual memorial meetings, inviting their friends to join in remembering one of their loved ones who had passed away. In such a meeting, they prayed for the progress of his soul, recounted his services to the Cause, described his qualities, recited Tablets revealed in his honour, if any, and offered charitable donations on his behalf. This practice of annual remembrance of the departed, which is not a binding obligation in the Bahá’í Faith, is now followed by many Bahá’í families. The organisation of such meetings is not usually the responsibility of the institutions of the Faith. They are arranged by individuals on the anniversaries of the passing of their loved ones.

Adib Taherzadeh, The Covenant of Bahá’u’lláh, p. 16-17

It must be noted that there is nothing in the Bahá’í writings either to condemn the celebration of one's birthday or to encourage it.

Adib Taherzadeh, The Covenant of Bahá’u’lláh, p. 17