Monaco's kind climate, favoured by the famous for over a century, means that there isn't a "bad" time to visit. July and August will see temperatures cruise above 30C, and coupled with the huge crowds of tourists this can be an uncomfortable time to visit. Spring and Autumn are arguably the perfect times to go, warm enough to visit the beach but outside peak season. Even in winter, when temperatures will struggle to fall below 10, it is usually mild enough to walk around in light(ish) clothing. The Formula 1 Grand Prix is held towards the end of May, and the races are held on the streets of Monaco.  Be aware this is a very busy time: hotels are expensive, the traffic is heavy, public transport is crowded and grandstands obscure many of the famous views.  If you are not a fan of Formula 1, avoid this period and save on the stress.