"On the tenth day of the first month the people went up from the Jordan and camped at Gilgal on the eastern border of Jericho. And Joshua set up at Gilgal the twelve stones they had taken out of the Jordan." NIV translation
The tenth day of the first month was the day that was set aside for selecting the Passover lamb. This lamb was to be taken care of for four days until its sacrifice (see Exodus 12:3). We are reminded that, throughout the Bible, the number four is associated with trials and this was a reminder to the Jews that God would take care of them during their struggles in the Promised Land.
"He said to the Israelites, 'In the future when your descendants ask their parents, "What do these stones mean?" tell them, "Israel crossed the Jordan on dry ground." For the Lord your God dried up the Jordan before you until you had crossed over. The Lord your God did to the Jordan what he had done to the Red Sea when he dried it up before us until we had crossed over.'" NIV translation
Joshua told the people what they were to tell the future generations. He also reminds them that it was equal to what He had done when He brought them out of Egypt. We notice that it was all about remembering what God had done on their behalf. That was their testimony; what is yours?
"'He did this so that all the peoples of the earth might know that the hand of the Lord is powerful and so that you might always fear the Lord your God.'" NIV translation
As we see, God's purpose in doing this was for all mankind. As Christians, we all benefit from Israel's testimony of deliverance as it builds us up in our faith. For those who do not accept Christ, it is a warning that lets them know that it is not a good idea to mess with God's people. For Israel, it was a reminder to them that there is no other god like the Lord. They had entered a land where there were all kinds of idols and they were to remember the difference between an idol and the Lord God.