Twelve tribes reach the promised land. And are camped at their first stop Gilgal. In order not to forget the miraculous crossing of the Jordan, Joshua commanded the people to bring the twelve stones on which the priests had stood and built a memorial at Gilgal.
And command ye them, saying, Take yourselves hence out of the midst of the Jordan, out of the place where the priests’ feet stood firmly, twelve stones, and ye shall carry them over with you, ... and these stones shall be for a memorial unto the children of Israel for ever. (Joshua 4:3-7 [Leeser])