In the following sentence
I was to have left on Thursday. But on Thursday I had a terrible cold, so I decided to wait till Saturday.
I think was to have left by Thursday should be used.
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Sign up to join this communityThe original sentence is correct, and it says that the person had to leave on Thursday, but couldn't do so because he/she had a cold on Thursday.
If you use 'by Thursday', it means that he/she could have left on any day prior to Thursday, i.e., on Wednesday or Tuesday etc.