Date.DayOfWeek returning the wrong day number?

Thanks VahidDM.

That might be an immediate short term fix to the current instance of this report, however we have used this function in many other reports which have already been published. If the behaviour of the function has changed, all those reports would need to be ammended.

Also, if the function behaviour has changed, why do the constants (eg Day.Saturday) remain as before and not reflect the new order?

I looked more closely at the online documentation  
https://docs.microsoft.com/en-us/powerquery-m/date-dayofweek
and it refers to defaulting to the culture default if not explicitly stated as a firstdayofweek parameter. This is the case in our function calls.

Further reading indicates PowerBI takes the defaults from the regional settings of the device on which the PBIX was originally created:
https://www.sqlbi.com/articles/changing-the-culture-of-a-power-bi-desktop-file/

Following that line, it seems our device language settings were recently changed and the regional default for that language now sets the FirstDayOfthe Week to Monday.

So I now know the source of the problem and its scale I have only to 'fix' the regional settings for those reports developed under the new regional settings.

I am still not sure what impact this will have when published online given the default language of the host servers and the lanuage settings in the user's browsers. 

I will test it further unless someone can point me to some definitive documentation?

Thanks!