In standard reporting you can use the "Day, Week, Week" toggles to change the date grouping for the line chart and performance by date table.
The day, week, month toggles can be found on the line chart and performance by date table in the account overview and when you drill down. For example you can change the date grouping to "Month" in the performance by date table for a specific brand.
The day, week, and month toggles do not over ride the main date selector. For example if the main date selector is set to January 1st through May 12th, the May group will only show data for May 1st to May 12th. If you want the table to show all of a month you can adjust the main date selector in the top right hand corner to include whole months.
You can choose the 'first day of the week' in Settings. It is automatically set to Sunday. This change will be reflect in all reports and dashboards in your account. If another user has a different day set as their first day of the week, it will cause data, reports and dashboards to be displayed differently.
The day, week, month toggle can also be found in Reports and Dashboards and it works it in the same way.