The problem with discounting (X% off, Y for the price of Z) is that it trains customers to wait for the discount and to anchor on the discounted price as the “real” price.
The add-on alternative
Instead of “15% off mains Wednesday”, run “a free glass of prosecco with any main course Wednesday”. The cost to you is $3–$5. The customer perception is a $15 value. Your menu price isn’t touched.
The compounding effect
Customers who got the prosecco bought a second glass at 60%+ rate. The add-on triggers a paid follow-on purchase. The discount usually doesn’t.