Make this jelly in the fall, when crab apples are plentiful. It can then be canned and enjoyed throughout the year.
Wash and quarter apples; place in a 6-quart saucepan. Add 6 cups water; place over medium-high heat; bring to a boil. Reduce to a simmer; cook until very soft, 45 to 60 minutes; remove from heat. Pour into a bowl through a fine mesh strainer lined with two layers of damp cheesecloth. Gather cheesecloth; tie into a bundle. Suspend from a wooden-spoon handle set over bowl; drain, without pressing solids, 1 hour, to yield about 4 cups juice.
Place juice in a saucepan over medium-high heat; bring to a simmer. Cook for 10 minutes; skim foam. Add sugar; stir to dissolve. Clip on a candy thermometer; cook until the temperature reaches 220 degrees, skimming foam. Pour into jars, let cool, and keep refrigerated for up to 6 months.