Standard room not available for 1 night of stay

I want to book a 5-night stay, so I can get the 5th night free promotion. However, the 4th night of the desired trip doesn't have Standard room availability. Is there a way to get around this? If not, is there a chance a Standard room rate will become available for that date?

Thanks.

For reference, it is for a stay in October at Hilton Molino Stucky Venice.