September 22nd is exactly six months after setsubun.

Setsubun falls on the equinox, when the days and nights are of equal length. Humans consider it a holiday.

Festival of Origin

New Year's Banquet

Emperor Koumei Memorial

Empire Day 02/11

Vernal Equinox Festival (Setsubun) 03/20

Emperor Jimmu Memorial 04/03

Harvest Festival 10/17

Emperor's Birthday 11/03

But the day for the other equinox of the year is just treated as a normal day.

Winter

Spring

Summer

I can think of only one explanation for this discrepancy.

There used to be a season considered Spring's counterpart.

But now it's gone.

The world has three seasons - spring, summer, and winter.

But there were originally four, weren't there?

Seasons have a big effect on things like the weather and plants' lifecycles.

If one of them were to suddenly disappear...

The scenery around us would change, too. It would skip ahead to the next one. It's no wonder everyone feels as if something is missing.