Each year in late October, spectators from around the world gather on the banks of the Charles River in Boston — nervous and excited, waving posters and cheering — in celebration of the Head of the Charles Regatta, which enters its 55th year this weekend. Spanning three days, the biggest regatta in the world attracts the best rowing teams for a race, which starts at Boston University’s DeWolfe Boathouse and ends three miles upstream at the Eliot Bridge in Brighton.