Langebaan is an inland lagoon and deep water shipping harbour which is on the west coast of South Africa, about an hour's drive from the popular Bloubergstrand coastal windsurfing spots – or two hours' drive if you are coming out of Cape Town (the extra hour is due to traffic issues).
You can freesail in Langebaan harbour at the north end, off the town beach for example, or you can launch from the Cape Sport's centre which has excellent windsurf shop and hire facilites too with wind-free rigging on a grassy lawn (recommended). This northern end of the lagoon is very holiday orientated and capetonians often have second homes here.
There are some amazing other windsurf spots too, such as Kraal bay, inside the west coast nature reserve near a village called Churchhaven – and this is a wonderful flat water spot best visited at high water. Take a picnic or BBQ stuff as there are no shops.
For waves, try opposite the Langebaan harbour entrance as the Atlantic swell run straight in and set up a wave on the Club Mykonos Marina spending beach.
The main season at Langebaan is during the southern hemisphere summer (or European winter), with perhaps the best wind from January through to March. Langebaan sort of gets the same southeaster wind cycle as Cape Town from the months of October through to April, whereas in the other half of the year the wind blows in more temporarily with rain fronts, tracking in from the northwest.
Occasionally, when the wind goes westerly it's dank and cold and foggy there.
Google 'Cape Sports Centre' for more info. – although this is their quiet season right now.
September is known as the 'flower season' as this is when the vegetation that seems arid in summer springs into bloom after the winter rains.