6. Determining the day of the third birthday:

A. Child was born by Bein Hashmashos:[1]

If the child was born during Bein Hashmashos [i.e. after sunset but before nightfall[2]], then the Upsherinish should take place the next day. For example, if the child was born on the 3rd of Elul, during twilight, then the Upsherinish should take place on the 4th of Elul.

B. If one was born on the 30th of Cheshvan/Kisleiv, when does he celebrate his Upsherinish in a year that there is no 30th day in Cheshvan/Kisleiv?[3]

One who was born on the first day of Rosh Chodesh Kislev, which is the 30th of Cheshvan, and in the current year of his Upsherinish there is no 30th day of Cheshvan, and hence Rosh Chodesh Kislev is only one day, he is not considered three years old until Rosh Chodesh Kislev, the 1st of Kislev.[4] [This same law applies likewise regarding one who was born on the 30th of Kislev, which was the first day of Rosh Chodesh, and in the current year of his Upsherinish there is no 30th day of Kislev, he is not considered three years old until Rosh Chodesh Teves, the 1st of Teves.[5]]

Q&A If one was born on the 1st of Kisleiv, which was the only day of Rosh Chodesh, when does he celebrate his Upsherinish in a year that there are two days of Rosh Chodesh Kislev? If one was born on the first day of Kislev, which was the only day of Rosh Chodesh, and in the current year of his Upsherinish Rosh Chodesh Kislev is two days, he is considered three years old starting from the first day of Rosh Chodesh Kislev which is the 30th of Cheshvan.[6] However some Poskim[7] question this matter and suggest that perhaps he does not become three years old until the 1st of Kislev. If one was born on the 1st of Teves, which was the only day of Rosh Chodesh, when does he celebrate his Upsherinish in a year that there are two days of Rosh Chodesh Teves? This follows the same law as above.

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[1] See Nitei Gavriel 1:3 to initially do the next day, although one who desires may do so that day; See Igros Kodesh 11:5; 22:372 that the main Hakpada is not to precede it before the 3^rd birthday, while to delay it a few days is ok.

[2] See Nitei Gavriel 1:4 that those who follow Rabbeinu Tam may follow the previous day if born before sunset of Rabbeinu Tam

[3] Admur 55:13; M”A 55:10; Elya Raba 55:9; Yad Ahron; M”B 55:45; Ketzos Hashulchan 15:6; Derech Hachaim; Kneses Hagedola; Poskim in  Kaf Hachaim 55:60

[4] The reason: Although regarding the months and years we count starting from the second day of Rosh Chodesh, which is the main day [of the coming month] while the first day of Rosh Chodesh belongs to the previous month, nevertheless the child does not complete a full 13 years to the day until 29 full days have passed from Mar Cheshvan, just as it was when he was born, in which 29 days had passed from Mar Cheshvan. [Admur ibid; M”A ibid]

[5] Elya Raba 55:9; P”M 55 A”A 10; Machatzis Hashekel 55:10; Tehila Ledavid 55:8; Kaf Hachaim 55:60

[6] M”B 55:45 in name of Elya Raba; Bach; Rav Akiva Eiger; Ketzos Hashulchan 15:6

[7] Shaareiy Teshuvah 55:5 in name of Elya Raba 55:10 who states that perhaps according to M”A ibid who rules the 30^th day belongs to the previous month then one does not become Bar Mitzvah until Kislev begins.