Pelusin appears to be the name of a village somewhere within ancient Egypt, perhaps in the Land of Ramses. According to the interpretative Targum of Pseudo-Jonathan (which expands Exodus 19:4 with additional comments), the Israelites who were celebrating the first Passover in Egypt (see Exodus 12 in the Bible) were supernaturally transported to Mt. Sinai to properly celebrate this festival. After the celebration, they were returned to the "Pelusin" location within the borders of Egypt.
This legend has no basis in either the Bible's Old or New Testaments. One speculation is that later Second Temple period Rabbis were seeking to combine Moses request in Exodus 3:18b ( "please let us go a three days' journey into the wilderness, that we may sacrifice to the LORD our God.") with the events recorded later in the Passover account.