I saw in a popular book a suggestion that Avraham was born on Rosh Chodesh Tevet. Does anyone have any backing sources for this?
1See Gemara Rosh Hashana 11a: R' Eliezer holds he was born in Tishri, R' Yehoshua holds he was born in Nissan. (Exact dates not given)– MehMar 26, 2015 at 22:09
From Bereshit 17:26, Rashi seems to suggest that Avraham Avinu and Ishmael had the same birthday. בעצם היום הזה. שֶׁמָּלְאוּ לְאַבְרָהָם צ"ט שָׁנָה וּלְיִשְׁמָעֵאל י"ג שָׁנִים נמול אברהם וישמעאל בנו:– Eli83Oct 26, 2018 at 19:17
This page (it is too much to simply copy and paste) speaks of the medrashic concept that the righteous die on their birthdays [scroll down to citation 27 in the text]. As such, acknowledging that Avraham Avinu was righteous, and the gemara mentions possibilities for the Yahrzeit:
Avraham Avinu (2124 - 1638 B.C.E.:), according to some sources (Bava Basra 91a). [According to others, Avrohom Avinu's Yartzeit is 1 Tishrei, or Nissan [Moed Katan 28a].
one could work backwards and derive a birthday commensurate with the death date one accepts.
This answer deals with the sources for the yahrzeit as well.
3I'm not seeing any mention of the date of Avraham's passing in Bava Batra 91a. Your source appears to be unreliable.– MehMar 26, 2015 at 22:07