John Lightfoot on Matthew 9:14

[We and the Pharisees fast oft.] Monsters, rather than stories, are related of the Pharisees' fasts:--

I. It is known to all, from Luke 18:12, that they were wont to fast twice every week. The rise of which custom you may fetch from this tradition: "Ezra decreed ten decrees. He appointed the public reading of the law the second and fifth days of the week: and again on the sabbath at the Mincha [or evening service]. He instituted the session of the judges in cities on the second and fifth days of the week," &c. Of this matter discourse is had elsewhere: "If you ask the reason why the decree was made concerning the second and fifth days, &c., we must answer, saith the Gloss, from that which is said in Midras concerning Moses; namely, that he went up into the mount to receive the second tables on the fifth day of the week, and came down, God being now appeased, the second day. When, therefore, that ascent and descent was a time of grace, they so determined of the second and fifth days. And therefore they were wont to fast also on the second and fifth days."

II. It was not seldom that they enjoined themselves fasts, for this end, to have lucky dreams; or to attain the interpretation of some dream; or to turn away the ill import of a dream. Hence was that expression very usual, A fast for a dream; and it was a common proverb, A fast is as fit for a dream, as fire is for flax. For this cause it was allowed to fast on the sabbath, which otherwise was forbidden. See the Babylonian Talmud, in the tract Schabbath: where also we meet with the story of R. Joshua Bar Rabh Idai, who on the sabbath was splendidly received by R. Ishai, but would not eat because he was under a fast for a dream.

III. They fasted often to obtain their desires: "R. Josi fasted eighty fasts, and R. Simeon Ben Lachish three hundred for this end, that they might see R. Chaijah Rubbah." And often to avert threatening evils; of which fasts the tract Taanith does largely treat. Let one example be enough instead of many; and that is, of R. Zadok, who for forty years, that is, from the time when the gates of the Temple opened of their own accord (a sign of the destruction coming), did so mortify himself with fastings, that he was commonly called Chalsha, that is, The weak. And when the city was now destroyed, and he saw it was in vain to fast any longer, he used the physicians of Titus to restore his health, which, through too much abstinence, had been wasted.