Maastricht is the perfect city for shopping. The shops are open seven days a week with extended opening hours on Thursdays and have regular sales where you can benefit from massive discounts.
Sales in Maastricht are not planned for any particular period. In contrast with Belgium and Germany, sales in the Netherlands don't have a fixed start and end date. If you want to be sure when your favourite shop is holding its next sale, the best thing to do is call the shop and speak to an employee there.