You can redeem your promo code at the time of purchase. Promo codes cannot be applied to current meal subscriptions, they can only be applied to a new meal subscription. Promo codes can be applied on the checkout screen.
If you redeemed a promo for a specific dollar amount off over a specific number of weeks, your discount will only apply to consecutive weeks in which you receive a delivery. If you skip a week, your discount will no longer be valid.