To show performance "week over week" or "month over month" you will need to use the "Day, Week, Month" toggle buttons in the right-hand corner of the chart or table and set "date" as your category.
Tables, bar charts, combo charts, and line chart can all show data week over week and month over month. Grouping data by week or month can make it easier to spot trends and make comparisons.
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.