I believe one of the big impediments to people voting is that they don't get the day off from work paid. If anything should be a holiday, it's the day to elect our representatives.
While I don't believe the president can declare through executive order a permanent federal holiday, he can declare a one time federal holiday. The coming election day would be the perfect time to use that power.
It would set an excellent precedent and stand in stark contrast to Republican's throughout the country trying to suppress voting for purely partisan reasons.