Hi,
You'll have to first create a discount via the menu Orders>Discounts and configure it to start and end on the next Wednesday.
Once that's done, you can create a mass action with a trigger "every day" and configure a filter so that it always match with exactly one element. For example, it could be a user data mass action with a filter on the user_id being equal to the id of your super admin, which should hopefully never change.
Then, add an action "run MySQL query" with such a MySQL query:
UPDATE #__hikashop_discount SET discount_start = discount_start*604800, discount_end = discount_end*604800 WHERE discount_end > 1 AND discount_start > 1 AND discount_end < UNIX_TIMESTAMP();
That way, it will increase the start and end dates of the discount by the number of seconds in a week if the end date is smaller than the current date (which means that the discount period ended).
Note that if you have other discounts with start and end dates set in the past, this mass action will also update these, so you might want to add a condition like discount_code = "xxx" so that you target only one specific discount.