Question

fix the one word that is used incorrectly, everyday between the summer solstice in june and the winter solstice in december, there is a small decrease in the hours of sunlight.

Explanation:

"Everyday" is an adjective meaning ordinary, but here we need an adverb to modify the phrase about the time period. The correct adverb form meaning each day is "Every day".

Answer:

Change "Everyday" to "Every day"
The corrected sentence: Every day between the summer solstice in June and the winter solstice in December, there is a small decrease in the hours of sunlight.