I'm sure all Data Schoolers, that are not from the United States at least, have faced the same issue as me in Tableau regarding the week days at least once. I remember, I was trying to build a bar chart for a seasonal analysis. I wanted to see the weekdays sales performes on average and noticed something odd. The chart looked a little different than from what I anticipated. Take a look:
There's no doubt that this chart works, but something threw me off. Tableau decided, that the week should start with sunday instead of monday, what is what I am used to living in germany. This issue bugged me and I started to look into the reason for this.
I quickly indentified the issue and noticed that I have to change a parameter within the [Date] field from standart to ISO.
This was actually a quick and effective fix for the problem, but I was confused of what “Standard Gregorian” and “ISO-8601 Week-Based” really means. So I started to look further into it.
The Gregorian calendar was introduced by Pope Gregory XIII in 1582 and corrected a issue in the Julian calendar, which drifts further and further into the next years by being just 1 day too long. The goal was to realign the calendar with the solar year to maintain the accuracy of religious observances. It is the most widely used calendar globally today and was adopted for civil and international purposes across most countries, including Europe, the Americas, Asia, and Africa. The Gregorian calendar week starts on Monday or Sunday, depending on the region. And the specific region where Tableau is from (the US) has the week start on Sunday.
On the other hand we have ISO 8601. This is an international standard for representing dates and times, established by the International Organization for Standardization (ISO) in 1988. It was created to eliminate ambiguity in date formats across different countries and systems, ensuring clear communication in global exchanges. The standard uses the "YYYY-MM-DD" format for dates and a 24-hour clock for times. According to ISO 8601, the week starts on Monday, which is consistent across all implementations of the standard. ISO 8601 is used in computing, business, and logistics to avoid confusion in date interpretation where differing date formats could lead to errors.
So this is why Tableau provides us with these two choices. On the one hand we have a international standart used for business and logistics and on the other hand the widely used standart that is most common for US inhabitants and some more countires around the world aswell.