Taanit, Chapter Two, Mishnah Ten
This mishnah deals with fast days that come into conflict with other various holidays.
1) They do not ordain upon the community a fast on Rosh Hodesh, on Hanukkah, or on Purim, but if they had already begun [a series of fasts and one of these days intervened] they do not interrupt [their fasts], the words of Rabban Gamaliel.
2) Rabbi Meir said: even though Rabban Gamaliel said that the [fasts] should not be interrupted he agrees that they should not complete their fasts.
3) And the same applies to the Ninth of Av should it fall on Friday.
Section one: According to Rabban Gamaliel, the court may not decree that a series of fast days begins on any of these holidays. However, if a series of fast days had begun and then one of these holidays landed on a Monday or Thursday on which people were supposed to fast, they do indeed fast.
Section two: Rabbi Meir says that when Rabban Gamaliel said that Rosh Hodesh, Hannukah and Purim do not interrupt if the series of fasts had already begun, he nevertheless agrees that on these days they should not complete their fasts until the evening. In this way people will signify in at least some way that the day still has some celebratory character to it.
Section three: In the days in which the monthly calendar was not predetermined, the Ninth of Av (Tisha Bav) could fall on Friday. Today this cannot happen. If this happens they do not complete the fast, due to the honor accorded to Shabbat, so that they shouldnt begin Shabbat with ravenous hunger. Another reason seems to be that there is a mitzvah to add on to Shabbat at both the beginning and at the end. If one fasts during this period he is diminishing the joy of a time of day when he should already be celebrating.