GGB By planning it, not reacting to it. In our experience the reliable levers are a summer-adjusted roster with annual leave deliberately scheduled into the quiet months, a cash reserve parked during peak season sized to cover the trough shortfall, a harder look at every discretionary cost, and a delivery-leaning offer while footfall is low. The operators who struggle are usually the ones who spent the winter surplus as if winter was normal.
From Ramadan and the Seasonality Map: Revenue Planning for GCC Restaurants — the full reading, with the method behind this answer.