Taanit, Chapter Three, Mishnah Nine
This mishnah discusses what happens if they begin to fast and then it rains on the fast day.
1) If while they are fasting rain falls:
a) If before sunrise they do not complete the fast,
b) If after sunrise, they do complete the fast.
2) Rabbi Eliezer says: if before noon they do not complete the fast, if after noon they do complete it.
3) It happened that the rabbis decreed a fast in Lod and rain fell before noon. Rabbi Tarfon said to them: go, eat and drink and make a holiday. They went and ate and drank and observed the day as a holiday and at evening time they came and recited the Hallel Hagadol.
Section one: The fast actually begins at sunrise. Therefore if it rains before sunrise then the rain has begun before the fast and there is no reason to fast. However, if it rains after sunrise they must continue and complete the fast.
Section two: Noon is when most people eat their mid-day meal, the main meal of the day. One who does not eat until noon is not really fasting, even though he may not have eaten. Therefore, according to Rabbi Eliezer, if it rains before noon it is as if they have not yet begun the fast and they do not have to complete it. If it rains after noon they must complete the fast.
Section three: In this section we have a story that illustrates Rabbi Eliezers halakhah and goes even further. Not only did they end their fast, but they went out and celebrated the arrival of the rain. In the evening they recited Hallel Hagadol, which is Psalm 136, which contains the line He provides food for all living creatures, a line especially significant on the day when it begins to rain.