Wash the elderberries and put them dripping wet with stems in a large pot. Cover and simmer at medium heat for 20-25 minutes. Peel and quarter the apple and remove the core.
Cut the apple quarters into small pieces. Pass the elderberries through a fine sieve or cloth. Measure out 1 litre of juice. Put juice and jam sugar in a pot and mix well. Add apple pieces and Calvados.
Bring everything to the boil over a high heat, stirring constantly, then let it boil for 1 minute until it becomes bubbly. Remove the pot from the heat and pour the jelly hot into prepared glasses. Close the glasses, place them on the lid for approx. 5 minutes, turn them over and let the jelly cool down until it has become completely firm.