It's classic revenue management. The revenue manager has placed an "MS4" and/or "MX2" on Monday arrivals - minimum stay, 4 nights and/or maximum stay, two nights.

Evidently their demand curve is showing sufficient demand for longer stays so that they can do this and still sell out. And a hotel always prefers to sell lots of long stays because stayovers cost less per day to service than complete room turns.

Generally, you can circumvent this by booking the longer stay and then modifying your arrival date. Keep in mind, hotels are learning how guests do this and are becoming more aggressive about penalties and changes in rate.