The law, with its profound insight into the human psyche, required that the couple stay together during the first week to continue to celebrate. They are not to go to work or otherwise to separate, unless there is an urgent need. This tradition harks back to Jacob, who celebrated seven days with Leah and then with Rachel. The custom became widespread and was not confined to Jews; Samson’s marriage to his Philistine wife was also celebrated for seven days.
The law considers the seven-day rejoicing to have been formally instituted by Moses, much as he ordained the shiva, the seven days of mourning.