When is the best time to lift dahlias before a frost or after?

Always after the first hard frost blackens the foliage as this triggers the dormant mechanism. if you lift them too early the tubers will not have matured or will try to grow in a mild spell. Try to lift them fairly soon after the leaves go black, cleaning off as much soil as possible. Allow to dry thoroughly before storing. Most people have their own way of storing such as in a cardboard box covering them in dry peat, or individually in brown paper bags. Store them in a cool dry frost free place, ensuring all damaged parts are removed. Make sure the tubers don't touch. Dust the tubers with an anti fungal dust or sulfur powder to stop mold growing.