The October 19, 1987, "Black Monday" stock market crash occurred after Ronald Reagan's Secretary of State George Shultz had travelled to the Middle East to work on starting peace negotiations between Israel and its neighbors, which also included discussions about the Palestinians. This also corresponded to the Shemitah cycle on the Hebrew calendar. The stock market collapse led to a loss of approximately $ 500 billion in the United States and around $ 1.7 trillion globally. Here is some relevant information from President Reagan's radio address that was aired two days before the stock market collapse.
Excerpts from Ronald Reagan's Presidential Radio Address on October 17, 1987
Source: https://en.wikisource.org/wiki/Presidential_Radio_Address_-_17_October_1987
My fellow Americans:
As I speak to you, Secretary of State George Shultz is on a diplomatic mission that reflects the breadth, the intensity, and the importance of our country's foreign policy efforts. Today he's in the Middle East. He'll meet with the leaders of Israel, Jordan, Egypt, and Saudi Arabia. ... Our wider role in the Middle East - perhaps more than in any other region - is that of peacemaker. We are doing our best to help narrow the differences between Israel and her Arab neighbors so that real negotiations for peace can get started. The desire for peace and the will to make peace are growing in the region. Our job is to help. In the Persian Gulf we play a similar role. Along with the initiative in the United Nations Security Council, we are seeking a peaceful resolution of the Iran-Iraq war - one of the great tragedies of our time. A vigorous diplomatic effort is essential, and that is what Secretary Shultz' mission is all about.
After conferring with key leaders of the Middle East, he heads to Moscow. In his talks there, he will bring up the Arab-Israeli conflict and the Gulf war, as well as other conflicts like Afghanistan, Central America, Angola, and Cambodia. We've made it clear that if our two countries are to have better relations we must see a change in Soviet actions in these regional conflicts.