I thought that no matter what the shluchei bais din needed 15 days to get the word out of the day of rosh chodesh to the furthest places. So no matter what they knew which day chanuka was. Obviously they wouldn't know which day rosh chodesh teves was, but they could still hold 8 days chanuka?
thx
md
The Pnei Yehoshua asks a similar question and explains that even though most people know what day the month started on by the 15th of the month (as explained in Sanhedrin 41b), a minority still do not know exactly what date Rosh Chodesh fell on and they might mistakenly make Chanukah later. For this minority, the Sheluchim used to go.
Yitzhak Yeranen (Edus 2:4) suggests that the assumption that people know the date after 15 days applies only to witnesses who have to be precise about the date in the month, but other people would not have such a precise knowledge, and for them the Sheluchim used to go and spread the word.
The Chida (Pesach Einayim), however, suggests that the Sheluchim only reached the Euphrates in 15 days, but there were other far-flung communities farther away which would take longer to reach, and therefore the Sheluchim needed to spread the word, hoping that it would reach these communities by the 25th.
The same question could be asked about the Sheluchim going in Elul for Rosh Hashanah, which is 29 days later, so these answers would apply there as well.
Yoel Domb