Beginning in May and continuing through the summer, the Jehovah’s Witnesses will hold 381 District Conventions in 98 cities around the country. These three-day regional assemblies are estimated to involve some 1.7 million attendees this year. For many of the host cities, the conventions provide an economic boost as well as help with facility maintenance.

For instance, in Tucson, Ariz., a total of nearly 40,000 people will attend Jehovah’s Witnesses weekend conventions throughout the summer. The city’s convention and visitor’s bureau estimates each weekend convention will mean a $3 million economic impact for the city—especially welcome during the slow summer tourist season.

The convention brings another benefit: Each year the group cleans the Tucson Convention Center Arena as part of its arrangement with the city. This year a group of volunteers also painted the TCC arena steps to make it easier to see them.

Other cities around the country benefit in a similar way. In San Diego in May, for example, more than 6,000 Jehovah’s Witnesses volunteers showed up to clean Qualcomm Stadium in preparation for the group’s annual convention in the city, according to a news report in Voice of San Diego. It’s a tradition that goes back nearly three decades. The arrangement keeps costs low for both the city and the convention organizers.