Changing the "Last Week" timeframe from being Sun-Sat to Mon-Sun

I don't know if you can edit the default, but you can create a filter that will accomplish what you need. 

1. Add the Date filter you want. 
2. Click the Filter, and then at the bottom of the Date menu, click Edit date ranges.
3. On the Date Range Submenu, click Advanced.
4. Then set up your date filter as pictured. The dates I chose were Monday the, Jan 11 to Sun, Jan 17. Then you add a repeat pattern. 

Hello Jason! You have unfortunately found a Bug that has been acknowledged by Zendesk yet unfixed for years. This issue impacts all ZenDesk Explore users and ZenDesk displays false data to users when using the "Display Selected Dates" which will show a mismatch of date ranges. It is suggested from Zendesk to work around this, as it is still not prioritized as an issue to resolve.

I am eagerly awaiting Zendesk to either fix the filter or update it to say expected behavior is Mon-Sun... but for now...

To work around this I suggest creating a Custom Attribute using the formula below, this will display weeks grouped Sunday - Saturday, adjust the formula for the attribute you need (solved, updated, created, etc):

Name: Ticket Created - Sunday to Saturday
Formula: LEFTPART(START_OF_WEEK_SUNDAY([Ticket created - Date]),10) 

An alternative workaround is to set the "Last Week" filter then set it back to "Custom". This should preserve the TRUE date range (Sun-Sat) of the Last Week filter but will no longer be set to the Mon-Sun timeframe.

This introduces the risk of user error not picking Custom and extracting poor data.