The Ritual of Osiris
A useful clue to the original nature of a god or goddess is often furnished by the season at which his or her festival is celebrated. Thus, if the festival falls at the new or the full moon, there is a certain presumption that the deity thus honoured either is the moon or at least has lunar affinities. If the festival is held at the winter or summer solstice, we naturally surmise that the god is the sun, or at all events that he stands in some close relation to that luminary. Again, if the festival coincides with the time of sowing or harvest, we are inclined to infer that the divinity is an embodiment of the earth or of the corn. These presumptions or inferences, taken by themselves, are by no means conclusive; but if they happen to be confirmed by other indications, the evidence may be regarded as fairly strong.
Unfortunately, in dealing with the Egyptian gods we are in a great measure precluded from making use of this clue. The reason is not that the dates of the festivals are always unknown, but that they shifted from year to year, until after a long interval they had revolved through the whole course of the seasons. This gradual revolution of the festal Egyptian cycle resulted from the employment of a calendar year which neither corresponded exactly to the solar year nor was periodically corrected by intercalation.