Rosh Hashanah, Chapter Two, Mishnah Two. In the previous mishnah we learned about the Tzedukim - those people who did not believe in the Oral Torah - who sought to confuse the Sages and mislead them as to which day Rosh Chodesh fell on, and would hire false witnesses to testify that they had seen the new moon.
The second mishnah continues along the same lines and tells of the method that was used to inform the communities of the Diaspora which day Rosh Chodesh fell on. This method worked almost instantly, but it too was tampered with and corrupted.
The words of the mishnah:
"Barishonah hayu masi'in mas'uos" - originally they would light torches on the mountaintops, and in this way they informed the Diaspora with great speed, within a few minutes, when the month had been sanctified.
This was the order of things: the torches were lit only when Rosh Chodesh fell on the thirtieth day of the previous month, that is, when the month was twenty nine days long. The fire was lit on the night following the sanctification, the night between the thirtieth day and the thirty first day.
"Mishekilkelu haKusim":
The Kusim - they are the Samaritans - also did not believe in the Oral Torah. They would light torches even on days when the beis din had established Rosh Chodesh on the second day, and once this corruption was introduced the torches became unreliable.
"Hiskinu sheyehu shluchin yotz'in":
Once this problem arose, the Sages had to institute a new arrangement: authorized messengers would go out to the Diaspora to announce which day Rosh Chodesh fell on. Naturally, this method took a long time.
From here arose the need for the communities of the Diaspora to keep two days of Yom Tov, since the messengers could not arrive in time, and the people of the Diaspora did not know on Yom Tov itself which day had actually been Rosh Chodesh.
In summary: in this mishnah we learned about the two methods of informing the Diaspora about the sanctification of the month: at first by lighting torches on the mountaintops, which conveyed the news swiftly; and once the Kusim corrupted the system and lit torches even at the wrong times, the Sages instituted that messengers would go out to the Diaspora. Because of the delay involved in this mission, the communities of the Diaspora became obligated to keep two days of Yom Tov.