“fast often.” The Mosaic Law only required fasting one day a year; the day of Atonement. Leviticus 16:29 (KJV) says, “And this shall be a statute for ever unto you: that in the seventh month, on the tenth day of the month, ye shall afflict your souls, and do no work at all, whether it be one of your own country, or a stranger that sojourneth among you.” The idiomatic phrase, “afflict your soul,” was known to refer to fasting. Nevertheless, fasting became a regular practice for many people.
The first fasting in the Bible is when Moses went up on Mt. Sinai for 40 days and nights and did not eat or drink (Exod. 34:28). Often a fast was just from sunrise to sunset, much like Muslims do today during the month of Ramadan, and after sunset the person could eat (Judg. 20:26; 1 Sam. 14:24; 2 Sam. 1:12; 3:35).
Fasting was done for many different reasons, but most often as a demonstration of humility toward God, and to get His favor. Of course, for some religious people, including the Pharisees, part of the reason for fasting was so that others would see and be impressed (Matt. 6:16; 23:5). We learn from history, and from the example of the Pharisees, that the Pharisees fasted twice a week, Monday and Thursday (Luke 18:12). By the second century there were Christians who were fasting twice a week, but they chose days when the Jews were not fasting (Didache 8:1).
In contrast to fasting, feasting was common also. Feasts were generally held for an important occasion: weddings; the weaning of a child; the arrival or even the approaching departure of guests; sheep shearing time; the weekly Sabbath, which was considered a joyous occasion; the sighting of the new moon, which started the new month; and of course the Feasts in the Law such as the Feast of Unleavened Bread, and the feasts such as Purim that post-dated the Law.
The life of a Jew who loved God was one that showed great dedication to God, and also showed a great love of life and enjoyment of what God had created.
On a textual note, there is some question as to whether the word “often” was in the original text of Matthew. Some Greek manuscripts include it and some do not have it. The Shem Tov Hebrew text has it, and that along with the Greek manuscript evidence is why the REV includes it.