Equinoctial periods, because of their balanced nature, are fit to bear sanctity. Therefore, these periods contain most of Israel's Festivals. However, there are other times of year less sympathetic to Israel - periods marked by poles in cold or heat.
One should not use fast days for leisure or for field trips. One should even refrain from performing permissible acts such as showering or cutting one's hair. If a person spends the fast indifferently, he has not properly fulfilled his obligation.
After the destruction of the First Temple, the Prophets and the Sages of Israel legislated fasting on the Tenth of Tevet, for it was on that day that Nebuchadnezzar, the King of Babylon, and his legions placed Jerusalem under siege.
In the past few years, nations have come upon us and demanded that we relinquish portions of the Land of Israel to strangers. This is one of the calamities concerning which we are supposed to fast and cry out to God about.
After the First Temple was destroyed, the Prophets instituted fasts marking the tragic events surrounding the Destruction and the ensuing exile of the Jewish People.
This step was taken in order to prompt the nation to grieve and mourn over the Destruction and Exile.