Translation:Are you free on Saturday?
Sometimes they focus too much on 'meaning' translation, making it harder to test. "Are you free on Saturday" is not a direct translation. It should be "Do you have free time on Saturday". In this case, a direct translation would be preferred and easier to understand for english speakers.
I was trying to understand why your answer was downvoted.
Then I realised that days of the week are related to the Julian calendar. Older Chinese texts would refer to days of the month (lunisolar calendar - often just called a lunar calendar) and did not need days of the week at all. There have been a lot of iterations in between of course, including some trial of 10 day weeks etc, but ultimately week days are not a big part of longer term Chinese history.
The idea of naming days of the week after the planets is a ‘western’ thing - think Roman or Greek (it’s possible the 7 day week comes from Babylon but naming each of the days of the week after planets was more Roman/Greek?). They do exist in Chinese, Japanese and Korean but only because they are derived from / refer to calendars which use weeks.
This sentence doesn’t differentiate. It could mean either.
If you wanted to make the distinction you could say 你这个星期六有空吗？(Are you free this Saturday?) vs 你通常星期六有空吗? (Are you usually free on Saturdays?)
[NB I’ve personally heard 常常 (often) used here more than 通常(usually) in conversations I’ve had]
But generally it’s ok to be a bit ambiguous. Context and further discussion cover it.