Published in 1961, Gaster's magnum opus still has relevance today. Entitled, Thespis, this book presents the thesis that all festivals are owed, initially, to seasonal concerns. Based upon the work of other scholars who dealt with the English mummers' play, the Rig Veda, ancient Chinese folk songs, the Grail romances, the Scandinavian Elder Edda and Greek tragedy, Gaster applies the same formulae to an appreciation of cultic festivals in the ancient Near East. His argument: that there are four types of festivals, falling into two clear categories.
The first category is that of Kenosis ('emptying'). This category contains the following two rites:
1) Rites of Mortification: An example of this would be the festival of the 9th of Ab (תשעה באב). This was originally a festival to the Babylonian god of fertility, Tammuz. Occurring in the driest part of the Palestinian year, the festival aims at encouraging rainfall through encouraging tears. Such rites were common throughout the region, making the Psalmist's declaration that "he who sows in tears will reap with joy" (Ps 126:5) a pertinent one indeed.
2) Rites of Purgation: An example of this would be Yom HaKippurim. It is common for rites of purgation to precede rites of jubilation (dealt with shortly), and Gaster refers to the contemporary practise amongst Orthodox Jews of fasting on their wedding day.
The second category is Plerosis ('filling'). This category contains the following two rites:
1) Rites of Invigoration: These rites celebrate the onset of the harvest and are frequently marked with overt sexual practises. Passover is the classic example, an indication of its sexual nature lying within the circumcision performed by the nation prior to eating the paschal sacrifice (Jos 5:2-11), as well as the association between Passover and Song of Songs.
2) Rites of Jubilation: Once the harvest is assured, festivals take on a greater sense of celebration. An example of such a festival is Shabuot, not to mention Sukkot (which follows on from Yom HaKippurim, as mentioned) - the first day of which is referred to as being the most joyful day in the Hebrew calendar.
Every one of these festivals also possesses an added historical reason. The 9th of Ab commemorates the destruction of the two Temples in Jerusalem; Shabuot celebrates the reception of the law at Sinai; Passover celebrates the exodus from Egypt; etc. These (pseudo-)historical reasons are supplementary to the festival, and were associated with the festival at such a time as the Israelites saw fit to distance themselves from the cultic practises of their forebears. While they were not able to eradicate the observance of the seasonal festivals from popular consciousness, they were able to subvert them. While it may come as a surprise to some, the celebrations of our world lie steeped in purely practical (not to mention, climatic) concerns.