Likutei Moharan 179:2/ליקוטי מוהר"ן קע״ט:ב׳

12. This is as our Sages teach: “The fast of the fourth month, and the fast of the fifth month… will become celebration and joy” (Zechariah 8:19). How can it be called both a fast and celebration? [When there is peace, there is “celebration and joy,”] but when there is no peace, there is “a fast” (Rosh HaShanah 18b). That is, when instead of peace there is strife, then “a fast”—fasting is necessary, as above. And by fasting, peace is made.

2Then, “when there is peace, there is ‘celebration and joy.’” This is because by fasting, the structure and vitality of joy is constructed. For the value of fasting is that it rouses and revives the dead—i.e., the days spent in darkness that have no vitality.

3This is because drawn into each and every day is an influx of bounty from Above. On a day a person performs mitzvot and good deeds, he then gives life to that day and draws into it vitality and a great influx of bounty from Above. But if, God forbid, he does not perform mitzvot, then the influx of bounty from Above descends only in a very, very limited manner—only enough for restricted survival.

4For when he takes that day and performs evil on it, God forbid, he then diminishes, nurtures from, and sucks up the limited influx of bounty which that day has. Eventually, he also sucks up the essence of the vitality of the day itself, aside from the influx of bounty which routinely comes from Above.

5For each day is a creation, with its individual vitality aside from the influx of bounty. Yet he also sucks up the day’s individual vitality until these days are left dead. This is analogous to an infant who nurses from his mother’s breast. As long as she has milk, he continues to suckle the milk. But when the milk stops, he sucks up her blood and her very vitality.

6However, by fasting, a person rouses and revives these days—it all depends on the fasting. This can be understood from the physical realm. When a person fasts, he consequently has no vitality and strength from that day, since then he does not eat or drink. Nevertheless, he serves God. In that case, he serves God with the strength that he has from the previous day. Were it not so, from where does he have the strength? We find therefore that he serves today with yesterday’s strength, and thereby brings vitality into that previous day.

7And afterwards, when he again fasts—in which case the previous day’s strength is not enough for him because the body has already been weakened—he must then go even further back and make use of the preceding days’ strength. Consequently, he each time brings vitality and illumination into the earlier days which fell and died. It is even possible to fast so much that he will have to make use of the strength of the days during which he nursed from his mother’s breast. Then, he revives and illuminates all the days.

8Therefore, by fasting, the structure of joy—the vitality and core of all the mitzvot, as explained elsewhere—is constructed. {This matter was not explained well. The point is that by fasting—by removing the strength of all past days and incorporating it into the service of God, thus reviving all the dead days which he blemished—he thereby constructs joy, as above.}

9This is as it is written (Psalms 90:15), “Make us rejoice according to the days You afflicted us.” The word “ANiTanu (You afflicted us)” is similar to TAaNit (fasting). In other words, the joy should be “according to the days of fasting.” For commensurate with the days of fasting, so does he revive the earlier days with his mitzvot and good deeds. And accordingly joy is constructed, as mentioned above.

10This is what the Sages teach: When there is peace, there is “celebration and joy.” For when peace exists—i.e., by fasting, as above—as a result there is celebration and joy. This is as in, “Make us rejoice according to the days anitanu”—commensurate with the taanit is the joy.