Many Seders serve eggs at the start of Shulchan Orech. Reasons and lessons of this custom are varied and numerous. There is no old, primary source for the custom, as far as I know.
In last Wednesday's NY Times Dining section there was an article about a new trend of urbanites owning chickens and using the eggs. Towards the end of the article was this gem:
In the last month, backyard chickens across the country have begun laying again. Left to their own rhythms, hens slow down or stop laying eggs altogether in the winter, because their reproductive cycle is linked to daylight. For centuries, the simultaneous return of eggs and the sun was seen as a quasi-magical coincidence; it is no wonder eggs are central to ancient spring celebrations like Easter, Passover and Nowruz, the Persian New Year, which begins on the spring equinox.Makes sense to me.
As a particularly meaningful way of celebrating the return of the eggs, they were incorporated into the Seder. It's a classic example of adding religious meaning to something that was done by all.
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