Before the Kuthites sabatoged the process, the Sages used a torch relay to let the Diaspora know when a New Moon had been sanctified. Fire signals were waved from the top of Mount Olives until the person on the next mountain did the same, and, in this way, the message was passed from one mountaintop to another, until it reached Pumbedita.
Tractate Rosh Hashana (23A) discusses when these signals were used, and concludes they were only used when the new moon appeared on the 29th day of the month. Why? Doesn't really matter. Trust me, it's confusing. What's facinating though, is the way Rashi glosses the Talmud's answer to a suggestion from a member of the Talmudical peanut gallary who objected to the idea that only 29-day months were announced with the torch relay.
Suggestion: Let signals be lit for both full (30-day) months and abridged (29-day) months. When a 29-day month ends on Friday night, sure, you can't play with fire because of Shabbas, but that's ok: When people don't see a signal on Friday night or Saturday night they'll work out for themselves that it must have been an abridged month.
Answer: People are too clever for their own good. If they don't see a signal on both Friday night and Saturday night, some will think that it was a 30-day month, but "circumstances" prevented the signals from being lit on Saturday night.
Rashi on the word "circumstances": Drunkedness from the Sabbath feast.
How bizarre is that? Though it's hard to say what this really tells us about behavior in Temple times, clearly Rashi, writing from his 12th century pespective, thought that blowing off a critical Saturday-night responsibility because you got hammered at Shabbos lunch was a perfectly ordinary thing to do. Makes you wonder how they got anything done in the 12th century, if a straight-shooter like Rashi thought that your hangover was a good reason for screwing over the whole entire Diaspora.