In order that they should not tarry three days without Toroh - כדי שלא ילינו ג' ימים בלא תורה
Summary:
We establish the days of reading the תורה on Monday and Thursday, because those were the days (respectively) on which משה descended and ascended הר סיני for the third time
And we judge on Monday and Thursday - 'דנין בב' ובה
Summary:
עזרא instituted that every city should have a sitting בי"ד twice a week, where previously only one (large) city had a sitting בי"ד (every day).