By John Sanidopoulos
The Feast of the Conception of the Holy and Glorious Prophet, Forerunner and Baptist John is a feast upon which some of the other major feasts of the Church stand, since it is from the date of the conception of John the Baptist we can calculate when his birth took place (nine months later), when the Annunciation to the Virgin Mary took place (six months later), and when the Birth of Christ took place (nine months after the Annunciation).
We base all our information for these dates on the first chapters of the Gospel of Luke, where we read that Zechariah went to the Temple to offer incense as a High Priest on the Jewish feast of Atonement (Yom Kippur). We know it was the feast of Atonement because the Evangelist Luke tells us that he was alone in the Temple while everyone was outside praying, which is what the Book of Leviticus commands for the Day of Atonement.
This festival of Atonement was supposed to take place on a certain day each year. Leviticus 16:29-30 tells us: "This is to be a lasting ordinance for you: On the tenth day of the seventh month you must deny yourselves and not do any work — whether native-born or a foreigner residing among you — because on this day atonement will be made for you, to cleanse you." According to the reckoning done centuries ago by the Church when establishing the feast of the Conception of John the Baptist, the Day of Atonement took place on September 10th, which was the day the conception of John the Baptist was announced (this is commemorated in the Orthodox Church on September 12th). Five days after the Day of Atonement the festival of Tabernacles (Sukkot) began, beginning on September 15th and lasting seven days until September 21st (Lev. 23:33). The High Priest was required to be in Jerusalem serving during all these festive days.
With the festivals of Atonement and Tabernacles completed on September 21st, Zechariah made his journey back home on September 22nd. Since the Church wishes to be as accurate as possible when the conception of John the Baptist in fact took place, being the feast upon which other major feasts of the Church stood, it reckoned to the exact day that this must have happened on September 23rd, when he reunited with his wife Elizabeth. This is how Saint Nikodemos the Hagiorite explains the origins of the date of the feast of the Conception of John the Baptist.