The Palestinian Authority declared three days of mourning and closed Palestinian schools. Hamas official Ismail Haniyeh suggested, "This is the moment Sheikh Yassin dreamed about". The Hamas leadership said Ariel Sharon had "opened the gates of hell." Hamas called for retaliation against Israel. About 200,000 people took to the streets of the Gaza Strip for Yassin's funeral as Israeli forces declared a national alert.  The assassination of Yassin also led to the fact that Hamas, for the first time, named as the most popular movement in Palestine by the residents of the West Bank and Gaza Strip two weeks after the assassination.  Abdel Aziz Rantisi was announced as the new head of Hamas. At a memorial service for Sheik Yassin, he declared that "The Israelis will not know security... We will fight them until the liberation of Palestine, the whole of Palestine." Publicly addressing the "military wing" of Hamas, Rantisi suggested, "The door is open for you to strike all places, all the time and using all means." Rantisi was himself killed by Israel on 17 April 2004 in an assassination almost identical to that of Yassin. He was killed by three rockets fired from a Gunship by the Israeli Military.  On 31 August 2004, at least 15 Israeli people were killed and 80 injured in a suicide attack against two Israeli buses in Beersheba. Hamas stated the attack was a revenge for the assassination of Rantisi and Yassin. Following the bombing, an estimated 20,000 Hamas supporters in Gaza took to Gaza's streets, celebrating the successful attack.

Answer this question "What caused this closure?" by extracting the answer from the text above.
"This is the moment Sheikh Yassin dreamed about