The Gemoro says (bottom of 22b) that torches were lit to publicise Rosh Chodesh "LeOr Iburo", which Rashi explains means the night AFTER the "Iburo" - the day that can be either Rosh Chodesh or added to extend the previous month.
Does not "Or..." normally mean the night preceding (or starting) a particular day? (I am thinking of "Or LeArbaah Asar", and common usage on invitations to events etc.)
Kol Tuv,
Meir Eliezer Bergman, Manchester UK
"Or l'Iburo" would mean the night before the 30th. But "l'Or Iburo" seems to mean the night after Iburo. This seems to be in the intention of Rashi Pesachim 2b DH l'Or and 3a DH Or l'Shelishi. (Note that Rashi there changes the Girsa that we have, which is l'Or Shelishi, to Or l'Shelishi, since it is referring to the evening before the third day.)
Be well,
M. Kornfeld
Kollel Iyun Hadaf