The data displayed in our Google Analytics reports and graphs is obtained via the authorized connection for the website profile you connected to the campaign, using Google's API.
According to their support documentation
, a week is not always 7 days:
A week starts on a Sunday and ends on a Saturday. For example, if you select a date range from Sunday, January 1 through Saturday, January 14, you have two data points for the two weeks. Each data point includes the sum for the week. Partial weeks are represented if your selected date range does not begin on a Sunday or end on a Saturday, but are not highlighted on the graph.
For example, this is how it worked for the last 2 weeks of 2017 based on their method:
Week 52 - 24/12/2017 - 30/12/2017 (including) : 7 days. We put it on chart as Sunday - 24/12/2017
Week 53 - 31/12/2017 - 31/12/2017: 1 day. We put it on chart as Sunday - 31/12/2017
We switched to a different date type available in their API beginning in January 2018 to prevent graphs from displaying a partial week when the Week frequency is selected. Insight Graphs and Metric Widgets now display date data points based on the first day of a date range. For example, if your graph date range is Monday, January 1, 2018 to Wednesday, January 31, 2018 with a frequency of Week, then the data points will be displayed as Mondays.