I'm not sure it would make a difference, as to turnout that is. If you look at the turnout history, 1968 was the the last year this country had a 60% turnout. Only two years since 1936, 1944 and 48 this country had a 60% plus turnout. This was when there was only election day, no early voting, no weekend voting, everyone had to turnout on election day or not vote. Since then we've bent over backwards to get people to the polls, it hasn't worked. Why would make election day a federal holiday enhance turnout. We've had states that have went to mail in ballots, played with computer voting from home. Made election day a 3 week to 6 week process in most states with early voting. Still we don't approach what was the normal turnout of 60% plus between 1936-68.
I don't think making election day a federal holiday would make one iota of a difference. If one has 3-6 weeks to vote and doesn't, why would he show up on election day even if it was a holiday? I think there are other reasons why we can't match that turnout when there was just one day and one day only, election day.
I think if one identifies with one or the other major party, he is more likely to vote than not. From 1936 thru 1968 around 75% of the total electorate were Republicans or Democrats. Since 1968 there been a steep decline in those who affiliate with both major parties. From 75% down to 55% today. That corresponds to the rise in independents who aren't all that political active or pay that much attention. They also don't vote as much or as a percentage. In 2016 the democrats made up but 30% of the total electorate, but 36% of those who actually voted. Republicans made up 27% of the total electorate, but 33% of those who voted. Independents made up 40% of the total electorate, but only 31% of those who went to the polls.
Those who identified or affiliated with the two major parties were over represented when it came to those who actually voted. Independents, vastly under represented. Independents do not have the stake in elections that Republicans and Democrats do. Hence they are far less likely to show up and vote even when they have 3-6 weeks to do so. Making election day a federal holiday isn't about to motivate them. My two cents anyway.