It can be made a day ahead.  Cut all the veggies and store them in an airtight container. In a separate container, you can store the dressing. If the sliced veggies are kept longer in the refrigerator, they will lose their freshness. So I prefer making this salad a few hours before it is consumed. I like the cucumber and onion crispy in my salad. The salt in the dressing will extract the water from the veggies making it soggy. That's why I keep the vegetable and the dressing separately.