Idun keeps the apples that keep the gods young, and when Loki lets her be carried off by the giant Thjazi the gods age in a single season until she is brought back. She is renewal held in trust: without her, everything that is old shows it at once.
Modern Practice
Idun is called on for renewal, for the recovery of energy, and at the beginning of spring. Her practice is the apple and the season; her offering is to give the first of what has returned.