On Day 3, add the strained lemon-wedge liquid to the slices in the saucepan. Stir in the sugar and lemon juice and bring to a boil. Simmer over moderate heat, without stirring, until the marmalade darkens slightly, about 30 minutes; skim off any foam as necessary. Test for doneness: Spoon 1 tablespoon of the marmalade onto a chilled plate and refrigerate until it is room temperature, about 3 minutes; the marmalade is ready when it thickens like jelly and a spoon leaves a trail when dragged through it. If not, continue simmering and testing every 10 minutes until it passes the test, up to 1 hour and 30 minutes.