The celebration of Palm Sunday is mentioned in the bible within the gospels of Mark, Matthew and (I think) Paul. For certain Mark and Matthew. However, it wasn't until the 4th century when most Christians felt free enough to begin openly celebrating. Along with Easter and Passover- this celebration has been around for a long, long time. Directly tied to the beginning of Christianity. Though, as with most holidays we celebrate today, the exacts may have changed.
Though, from my research- most Palm Sunday celebrations have changed little in the past few centuries. It is very uncommercialized and sect specific, therefore- unhindered by the outside world.
at 4:59 PM on Mar. 30, 2010