I figured it out! For anyone else looking for the solution, here it is.
In the admin, select Themes, then edit. Switch to the HTML tab and select "Template." Add the text "default" to whichever tab you want to show first (see line 111 as it's added for me to the custom tab). Make sure you remove "default" from whichever tab was previously selected ("Schedule" on line 106 if never edited before).