Traditionally, you should harvest on the longest day of the year - 21 December. However, I like to keep an eye on the leaves and when they start to yellow and begin to dry off I gently uproot them with a fork. Be careful not to yank the leaves or you might rip the stalks off. Then simply brush off any dirt and hang them in a dry, airy spot inside. Like other produce that is stored for a long time, it makes sense to eat any damaged ones first and store the rest.