Not Sun Dae
Sun Day has nothing to do with Sunday, even though some years they may be one and the same day.
Neither does it matter that the name Sunday did come from sun and day.
Sun Day is a day to celebrate the sun. (You probably guessed that.) This does not mean you should worship the sun. No, it's not anything remotely close to that.
Sun Day celebrations can include trips to the sun, sun watching, and sun bathing, which will be more enjoyable at some latitudes than others.
Other possible activities for Sun Day are making drawings of the sun, photographing the sun, and guessing whether or not the sun will cause the sky to be red at sundown. The latter activity must be done at least an hour before sunset. No cheating.
People come up with new methods of celebrating Sun Day each year. Maybe you'll invent one of your own today. If you do, let us know about it, if it's something that other people could do too next year.
Even though this isn't Sun Dae, I suppose you could eat a sundae for fun anyway.