In a pitcher stir together the simple syrup, cranberry juice, and lemon juice. Add the tequila and cointreau and stir to incorporate. Add the lemon slices and fill with ice.
⅓ cup honey ginger simple syrup, 4½ cups cranberry juice, 1 cup lemon juice, 1½ cups tequila, ½ cup Cointreau, ice, 1 lemon