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That's more likely "often" rather than "only". The following thread might be somewhat helpful for your understanding of は.If I understand correctly, we only use はfor days of the week if the nature of the question is contrasting like the example you gave above: "I know you don't work on Sunday, but how about on Saturday?"
日曜日はお休みだと思いますが、土曜日は働きますか?日曜日働きませんでわ土曜日は働きますか?