In DragonVale for the iPhone and iPad, the Solstice Dragon is a limited dragon that can only be obtained two times a year during the summer and winter solstice.
To get a Solstice Dragon, you need to breed an Earth type dragon with a Blazing Dragon (blazing = fire + air) during the winter or summer solstice. In the Northern Hemisphere, the Summer Solstice is in the later part of June, and the Winter Solstice is in the later part of December, both around the 20th of the month.
Check out my DragonVale Breeding Guide for more dragon breeding combinations.