Well, it obviously started right around the time that Christianity was competing with the pagan religions, or in the early days before the Catholic Church was a Catholic Church.
Perhaps coopted is the wrong word, but any way you look at it, the pagan symbols have become very tightly tied to the entire celebratory package.
Think about some other holiday symbols that are very tightly tied to the celebration of the holiday, but which don't show up in the liturgy...
How about the Christmas tree?
A Germanic Pagan symbol that was, as Christianity began to move through that area of Europe, coopted as a way of explaining to the pagans the concept of everlasting life.
After all, the Germanic pagans were bringing pine trees into their homes in the winter months because they never died -- they were at all times inhabited by the spirts of the Gods, whereas with other trees, Alder, Birch, etc., the Gods obviously packed up and went South to winter over...
Here's a little blurb from this site...
http://www.intermirifica.org/easter/eastsymbol.htm"The origin of the Easter egg is based on the fertility lore of the Indo-European races. To our pre-Christian ancestors it was a most startling event to see a new and live creature emerge from a seemingly dead object. The egg to them became a symbol of spring. Long ago in Persia people used to present each other with eggs at the spring equinox, which for them also marked the beginning of a new year.[58]
In Christian times the egg had bestowed upon it a religious interpretation, becoming a symbol of the rock tomb out of which Christ emerged to the new life of His Resurrection. There was in addition a very practical reason for making the egg a special sign of Easter joy since it used to be one of the foods that was forbidden in Lent. The faithful from early times painted Easter eggs in gay colors, had them blessed, ate them, and gave them to friends as Easter gifts.
The custom of using Easter eggs developed among the nations of northern Europe and Christian Asia soon after their conversion to Christianity. In countries of southern Europe, and consequently in South America, however, the tradition of Easter eggs never became popular."
The early Chrisitan missionaries weren't stupid people. They were persistent, but not stupid. They knew that the one-God message that they were bring to polythiestic people was going to be very, very disruptive, and that they had to make logical, applicable connections to the symbolism that existed in that religion in order to make the new concept acceptable.
The easiest way to get yourself killed as a missionary is to go in strong armed and say "Everything you've been doing is complete and utter CRAP! Get rid of it all, it's an offense to God!" The locals tend not to care for that very much.
Even when the Conquistadors started sweeping through Central and South America the missionaries they brought along tried to ease the transition to Christianity by adopting a limited amount of local symbology. It was kind of tough, given that the local religions were often based on human sacrified, but it was successful enough that in South and Central America you have some really odd pre-Christian customs that have been blended in to the Christian practices.
