The Charles River Conservancy and the Charles River Swimming Club announced that they’ve combined forces to host Charles River Swimming Day, a daylong celebration where people can plunge into the Charles River at the “Arthur Fiedler Dock” along the Esplanade. Earlier in the morning, the CR Swimming Club will host its 11th annual 1-mile swim down the river. It’s the first time the two events will be held on the same day, and to top it off, Charles River Swimming Day will include an after party across the river in Cambridge, officials said.