The losing of our historical reference points

The Uniform Monday Holiday Act passed in the 1960's put an end to observing proper holidays on their days that they are to be observed, giving three-day holidays, instead of observing why these days were observed.

Monday's Boston Marathon is one event affected by this law.  Most people today do not remember that prior to the 1969 change, this event was originally held only on April 19, unless it was a Sunday, when it was held April 20.  Can anyone identify why the Boston Marathon prior to 1968 was always held on April 19 (or if it was a Sunday, then April 20)?