In a large bowl, mix the cucumbers, onion, and bell pepper. Set them aside.
In a saucepan, combine the vinegar, sugar, salt, mustard seeds, and celery seeds. Bring the mixture to a boil over medium heat, stirring until the sugar dissolves.
Pour the vinegar mixture over the cucumber mixture and stir well to combine.
Cover and refrigerate for at least 24 hours to allow flavors to meld.
Use as a condiment for hot dogs, burgers, or sandwiches, or as a topping for your favorite dishes.