Every now and then it can be useful to run a sale. Especially during a slump.
You'll get a quick infusion of cash, be able to clear out some inventory, and the activity can pull you out of your slump.
They'll work for most brands, though some might want to avoid discounting and spin it as a "value-add" sale instead (e.g. "free gift with purchase"). Same net impact, just a better customer perception.
Normally I recommend planning sales in advance but I lean towards more organization than most. If you're a more ad-hoc decision maker then throw up a sale every now and then.
And why not next week?
Make it for everyone, every person on your email list, or pick a segment you want to improve in Repeat Customer Insights and only tell them about it.
Don't feel like you can't run a sale except as specific times. Sales are a tool you can use whenever you think it would be beneficial.
Measure your customer loyalty
Measure the different levels of customer loyalty with Repeat Customer Insights. It uses various models to segment and grade your customers based on their behavior.