💡 Detailed Explanation
The correct choice is Option 2: 'Because it is the "Book Organizing Day".' The announcement specifies that while Mondays are standard holidays ('毎週月曜日が定休日'), the reason for the temporary closing tomorrow (Tuesday) is: '明日火曜日は「本の整理の日」のため、臨時休館とさせていただきます' (Tomorrow, Tuesday, will be temporarily closed due to "Book Organizing Day"). Therefore, Option 2 is the correct reason.