http://h4labs.org/siri-next-monday-isnt-christmas/
If you ask Siri now, it thinks next Monday is the 15th.
Tonight’s bug occurred on the Watch. It told me to close my Activity rings like yesterday, even though I closed them several hours earlier.
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Update
I wanted to add that Alexa and Google define “next Monday” as I do. I didn’t add that to my blog because the point wasn’t to try and criticize Siri. Ambiguity will always be a problem.