Mix the cranberry and apple juices together in a pitcher. Keep it in the refrigerator until you're ready to make the mocktails.
Optional Step: Sugar rim the glasses by running a sliver of lemon around the rim, then dipping it into sugar.
Fill four glasses halfway with ice and pour the juice until each glass is about 1/3 of the way filled. Top off the last 2/3 of each glass with seltzer. Leave a little room at the top for bubbles!
Garnish each glass with a sprig of fresh rosemary and a few cranberries.
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Notes
Chill the juices ahead of time! Not only will this save you time on the day of your party, but you'll use less ice. This means the mocktail won't be watered down, so it will have a brighter flavor.
Taste for sweetness before adding the ice and seltzer. Cranberry juice is quite tart, but the sweet apple juice does offset the tartness. Keep in mind that you will be adding seltzer, which will mute the flavor of the juices.
Splurge on garnishes - they really do add a festive touch!
Use crushed ice rather than cubes. It will keep your drinks colder for longer.