The Arrhephoria

The Athenian feast of the Arrhephoria, or "dew carriers" relates to the three daughters of Cecrops:

Arrhephoria was a feast among the Athenians, instituted in honor of Athena.

The word is derived from the Greek term Ἀρρηφόρια, which is composed of ἀρρητον, "mystery," and φέρω, "I carry". (Perhaps not, see this.) This feast was also called Hersiphoria, from Herse, the daughter of Cecrops, on whose account it was established.

Herse pursued by Hermes.

On the Athenian Acropolis two girls aged between seven and eleven were elected to live for a year at a time as arrhephoroi, tending the sacred olive tree and weaving, with the help of other women, the new robe for Athena. Proud parents commemorated their daughters' service by making dedications on the Acropolis. At the annual festival of the Arrhephoria the girls (according to Pausanias), placed on their heads what the priestess of Athena gives them to carry and neither the priestess knows what it is she is giving them, nor do the girls who carry it. In the city there is a sacred precinct not far from that of Aphrodite in the Garden and through it runs a natural underground passage. Here the virgins descend. Down below they leave behind what they have brought and take something else and carry it, veiled as it is. These two virgins are discharged forthwith and others are taken up to the Acropolis in their place.

Interpretation of the festival is difficult because of the lack of sources, but it is clear that the virginal arrhephoroi are chosen from the noblest families of the city and are deployed in a context of impregnation (dew), sexual power (Aphrodite and Eros), and birth (Erichthonios). The word "arrhephoros" etymologically probably means "dew carrier", which at first sight does not help. The arrhephoroi were charged with weaving the peplos (garments) for Athena.. . .

Archaeological evidence reveals that from near the Erechtheion a secret stairway led off the Acropolis past a small rock-cut shrine of Eros and Aphrodite, near which was the precinct to which they were going. They mythical associations of the arrhephoroi are with their starting-point the Erechtheion. Kekrops, the first king of Athens, whose tomb was in the complex, had three daughters, Aglauros, Herse, and Pandrosos. The mystery revolves around innocence, obedience, and fecundity. They were given a closed basket by Athena who forbade them to open it. One night Aglauros and Herse gave in to curiosity, opened the basket, and saw Ericthonios, the mysterious child of Hephaestus. Snakes also appeared out of the basket, and in terror the two girls jumped off the Acropolis to their deaths. The sanctuary of Aglauros lies at the foot of the cliff; it may have been the precinct to which the arrhephoroi descended. Pandrosos, who did not succumb to this fatal curiosity, has a shrine next to the sacred olive tree on the Acropolis itself. More: Wikipedia

Update (11.23.11: I recently came across this book, which goes into the subject in depth: