Handfasting was the original way in Medieval Britain that couples joined in what we know as matrimony. By handfasting (actually placing of the couples palms in a way to signify the infinity sign) a couple was joined for a year and 1 day whereafter they could renew the handfast or not as they chose. Since access to clergy was not all that easy this made a great deal of sense.
Today handfasting is used symbolically as a portion of a wedding ceremony. It is a lovely ancient (some say Celtic) tradition being kept alive.
I am glad I came across the term today. It is fun learning where things come from and why.