Holiday Fireworks Vendor/Availability

The Engineers' Explosive Extravaganza was an event celebrated on or near July 4th on North American servers, corresponding with America's Independence Day. The Holiday Fireworks Vendors are associated with the Engineers' Explosive Extravaganza, not the Midsummer Fire Festival per se, though the Engineers' Explosive Extravaganza has been merged into the Midsummer Fire Festival on those realms.
The Engineers' Explosive Extravaganza is not celebrated on European servers, instead, on European realms, the Holiday Fireworks Vendors appear on Peon Day (which is in turn not celebrated on North American servers.)