We may not decree a community fast on Rosh Chodesh, Chanukah, or Purim; however, if they had [already] commenced [a series of fasts], they may not interrupt [it]. This is the opinion of Rabban Gamliel.  R' Meir says: Although Rabban Gamliel said, they may not interrupt [the series], he agrees that they should not complete [the fast].  The same is true of Tishah B'Av which falls on the eve of the Sabbath.