To sign week, take your non-dominant hand and hold it out flat, with your palm facing up. Then take your dominant hand, its pointer finger sticking out of the fist you form, and run its back side once across the palm of your other hand, from the base of your palm to past the tips of your non-dominant fingers. It is like you are indicating that one day runs or multiplies several times throughout a week.
Teach your advanced toddler the week sign once they have grasped the concept of different days and can count basic numbers.
Teach this sign along with the other flash cards pertaining to the different days of the week and the passing of time, from day to night, then a new day again.