Weeks in Chartio are defined using the ISO week date system. A week is specified by the ISO year in the format of YYYY and a week number in the format ww prefixed by the letter W, and the weekday number, beginning with Monday and ending with Sunday. The first week of the year is the week with the year's first Thursday.
Chartio uses ISO weeks because it is an industry standard issued by the International Organization for Standardization to avoid misinterpretation of numeric representations of dates. Additionally, every data source type available in Chartio supports ISO weeks.
If you wish to change the start of the week to another day, you can do that through SQL Mode. This is only recommended for table charts, as x-axis date labels expect ISO weeks.