When did they go to the Yarden?
Radak (2), Malbim: The spies returned on Nisan 8, and on the next day, they went to the Yarden. They crossed it on the 10th (4:19) 1 .
Radak: Tradition this teaches that Moshe died on Adar 7, and Yehoshua told the people to prepare on Nisan 7. Without the tradition, one might have thought that Moshe died earlier, and Yehoshua told the people some days after the mourning over Moshe finished. Some say that we say "Tzidkascha" on Shabbos afternoon, for that is when Moshe died. The calculation shows that this is wrong; the wall of Yericho fell on Shabbos.
Why did they spend the night there [and not cross immediately]?
Malbim: It was so they would cross during the day, and the miracle would be open and everyone would see.