Weather in May
South Africa, being situated in the southern hemisphere, is moving into
the winter season in May. Winters are not as cold as in the northern
hemisphere though, with day temperatures still going up to 18+ degrees
Celsius during the day, and around 5 degrees Celsius at night in the Cape
Town area. Other parts of South Africa even experience higher winter
temperatures during the day, often as high as 26 degrees Celsius.
Cape Town is a small part of South Africa with winter rainfall (the rest of South Africa being summer rainfall region), so be sure to bring an umbrella and raincoat. Cape Town weather is influenced in winter by cold fronts coming from the South Pole which can easily last a week or more at a time.
Cape Town locals often refer to Cape Town as the city of four seasons in one day. It is quite possible to have (especially in May) rainfall, warm sunshine, and cool winds in one day. Don't be fooled by the sun when getting up in the morning, it can easily turn into a cold, windy, and even wet day without warning.
A good general description of the weather in Cape Town can be found at here.