Not every world has a nice, smooth orbit around its sun. In the case of this world, it follows a slightly irregular orbit that puts it slightly closer to the sun every four years. Not much, but enough to make some interesting, temporary changes to the agricultural sectors of the world's economies. This time is known as Perfect Equinox.
For the most part, this is a good thing, especially to the areas near the Landis Kingdom. Most edible crops tend to do better with the extra sun and heat, resulting in a predictable cycle of surplus. Others don't do so well, but on the whole, good overall. Because of this, Landis celebrates the Perfect Equinox Festival every four years at the castle. It's a time of early harvests, the sale of unique goods, fun entertainment, thrilling sport, and all sorts of other things one would associate with something called the "Perfect Equinox Festival". It's a festival. For Perfect Equinox.
That's not to say it's all fun and games around this time. For instance, you can't really get around the inevitability that such a strange celestial property of the world is going to involve some tweaks to certain practices of magic. Also, various cults and shady sects hold some of their shady and more various rituals under a Perfect Equinox sun. And, most noticeably, certain plants only grow during this time, such as the deadly ice rose, named as such for the translucent, ice-like appearance of its petals, the unnervingly cool feeling of the flower, and the grimly cool feeling of formerly-living bodies exposed to the poisons contained within it. Fortunately, this being a well-known problem, antidotes exist and are usually in ready supply in any population center whenever the ice rose blooms. The only real threat posed is if someone is poisoned by it a long way from a town or if someone were to engineer a variant of the poison to be resistant to known antidotes, but really, that latter case is just plain silly.
Incidentally, note that this property of the world does imply a Perfect Solstice in the southern hemisphere, as well as an Imperfect Equinox and Solstice on the opposite end of the cycle, but those aren't as interesting to our story.