Pour the apple cider into the slow cooker. Add the brown sugar and whisk until completely dissolved.
Add the cinnamon sticks, whole cloves, whole allspice, orange slices, lemon slices, and apple slices.
Cover and cook on LOW for 2 hours to allow the cider to heat up slowly and the flavors to meld together.
Before serving, discard the spices, cinnamon sticks, orange slices, lemon slices and apple slices. Add more orange, lemon, or apple slices for garnish.