There's more to Tetsudo no Hi than that -- it's also the time when JR sells a special once-a-year-only pass that lets you travel anywhere on JR for three days for ¥7,500. The catch? Like the Seishun 18, it's local trains only.
https://en.wikivoyage.org/wiki/Seish...t#Other_passes
There's also a JR West-only version for ¥3,000, valid for one day only.