Why the dates move every year
| Holiday | Set by | Range it lands in |
|---|---|---|
| Seollal (설날) | Lunar new year, plus the day before and after | Late January to late February |
| Buddha's Birthday | 8th day of the 4th lunar month | Late April to late May |
| Chuseok (추석) | 15th day of the 8th lunar month, plus the day either side | Early September to early October |
| New Year's Day, Liberation Day, Christmas and the rest | A fixed date on the solar calendar | The same date every year |
A lunar year runs about 354 days, roughly eleven days short of a solar year. So a lunar holiday slides earlier each year, then jumps back when a leap month is added.