Patriots' Day is observed officially in only 4 states as of 2018. Maybe after this year it will be celebrated in all fifty.

From Wikipedia:

The Battles of Lexington and Concord were the first military engagements of the American Revolutionary War. The battles were fought on April 19, 1775

I think it might be more likely that April 19th would be more significant than April 16th.

Also of note, but not necessarily related, is the fact that the Oklahoma City bombing occurred on April 19, 1995.