Place salt, sugar, lemon slices, vinegar, peppercorns, sage, thyme, bay leaves, garlic, and 4 cups of the apple cider in a large saucepan. Bring to a boil over high, whisking occasionally to dissolve salt and sugar. Boil over high, whisking occasionally, until very fragrant, about 1 minute.
Pour hot cider mixture into a very large stockpot or other large heatproof container; stir in very cold water and remaining 4 cups apple cider.
Use immediately, or store, covered, in refrigerator up to 3 days.
