The Hall of Isaac at the Tombs of the Patriarchs in Hebron, West Bank, was filled with thousands of Jewish worshippers — men and women — on April 23 in the intermediate days of Passover. Most came for a special mass singing event, where together they sang during morning prayers the verse “What will I return to the Lord,” from the book of Psalms.

Documented by drone cameras, amplified and accompanied by music, the event is part of the mass singing trend, Koolulam — which is a wordplay on "koolulu," the traditional celebratory ululation of Middle Eastern and North African Jews, and "koolam," which means "everyone" in Hebrew. The trend is taking Israel by storm and was certainly reflected in the mass singing at the Tombs of the Patriarchs.

Koolulam gathers thousands of strangers without musical backgrounds for a new arrangement of a familiar song in three-part harmony following a 45-minute lesson. But the music is actually not the main point. The event, which usually lasts about three hours, includes all sorts of social interaction exercises. The common singing and the final video clip are just the artistic product and the viral source of inspiration for other participants. The phenomenon, which started with local events with only a few hundred participants, has become over the past few months a social movement that has stirred hundreds of thousands, been viewed by millions on YouTube and drawn international attention.

In the past year, countless Koolulam events have taken place throughout the world. In early April, Canadian Prime Minister Justin Trudeau participated in one such event in Toronto that marked 70 years of friendship between Israel and Canada, where he sang ultra-Orthodox singer Matisyahu’s song “One Day” along with thousands of other people. The event that broke the record for the number of participants took place a month ago in Washington, DC, at the American Israel Public Affairs Committee's annual conference and included 18,000 participants. Last September, Asia Society gave the initiative a prize for its contribution to “changing reality.”