Kos Coronaios of the Southern Cape Astronomy Club said South Africans should be able to catch sight of the ISS from 5:51am to 5:57am on the morning of May 21 when it'll appear as brightly as the planet Venus does just after sunset.
The ISS will start to appear at about 10 degrees above the northwestern horizon. It'll pass "very close" to Jupiter and Saturn before disappearing towards the southeastern horizon.
On May 22, the ISS will pass by a little earlier and will appear a magnitude dimmer; it'll be visible from 5:05am and at it's highest point will be 45 degrees above the northeastern horizon. It will then disappear at about 10 degrees above the southeastern horizon at 5:10am.
Both passes will be visible to the naked eye with the ISS appearing as a bright light moving from one end of the sky to the other, travelling at a speed of 28,000km/h.
Coronaios advises that the best way to see the ISS is to step outside a few minutes before the time it's expected to appear and face the direction that it'll rise from. The light will get brighter and brighter, thereafter coming overhead and disappearing behind you.
With a brightness of this magnitude occurring only once in a while, it is definitely a sight worth waking up early for.