This solution does assume that the data in first_day_of_week directly correlates to the series data. If the two aren't the same size or in the same order, then you'd end up with labels applied to the incorrect data points. I also verified that you can't reference the x and y values in the same way when using the array option, which definitely complicates things.
Given these limitations, I think your best option is to construct the hoverTemplate array in an external query and then reference it from within the table settings. That would eliminate any issues with data access and really put you in full control! Let me know if you have any questions about implementing that.