My feeling is that if Monday is now in the past then you would tend to use the latter one, but if Monday is still in the future then you could use either. I don't think this is totally clear-cut though. " M onday" needs a capital.
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AnonymousBut was ,here,refers to the past not the futureIn the case when Monday is in the future, "was" refers to the past time when you were still expecting to go on Monday. The implication is that you are no longer expecting to go on Monday. "I was to have gone on Monday, but now it's been cancelled."