Question about fasting and a promise to God

Some years ago I made a promise to God to fast some days, and I did so, but one of the days I fasted was most likely a sunday, i did not know you must not fast on sunday, so, did I broke my promise there?

It is not a sin to fast on Sunday. It’s just that many people choose not to in recognition of the resurrection.

Try fasting on Fridays.

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