As a lot of people choose to spend time with their families on bank holidays, it's probably a good idea to cook something like the traditional Sunday roast, even if you are actually making it on Monday. For this time of year lamb is a good choice - spring lamb is very popular, and you can make it lighter if the weather is warmer by using some more Mediterranean style vegetables - eg peppers, tomatoes and courgettes, and roasting them, as well as some small new potatoes such as Jersey Royals. Asparagus is also in season now so you could start with that, or even have it as a side vegetable if you prefer.
Strawberries are also just in season now, so strawberries and cream is nice for dessert, or you could make a Victoria sponge and fill it with fresh strawberries instead of jam.
You can find more ideas here.