WARSAW, Poland (AP) — Taylor Swift warned fans who traveled to her concert in Warsaw on Thursday to expect loud sirens in the afternoon honoring a key World War II anniversary.

When the moment came, many of the fans who arrived for the concert also stood in tribute.

The Polish capital held observances to mark the 80th anniversary of the start of the 1944 Warsaw Uprising, a 63-day revolt by Polish insurgents after five years of brutal Nazi German occupation. The entire city stops and alarm sirens sound every year on Aug. 1 at the exact time when the revolt was launched.

“To the people who are coming to the concert on August 1st don’t panic if you hear sirens alarm about 5 p.m. It will be the 80th anniversary & planes!” Swift’s “The Eras Tour” posted on social media.

Thousands of ticket-holders, many who traveled to Warsaw from afar, were in the area of the stadium early ahead of the evening performance.

A Polish news site, Onet, also published an “important message for all Swifties who are going to the concert” explaining the significance of the day.

“We ask you to remain calm and not to panic. In this way every year residents pay tribute to the heroes of 1944. Those who will be at that moment already outside the stadium, please remain quiet and get up.”

While some swifties joined the solemn commemoration, their mood was celebratory, as the countdown began to the singer’s performance.

Swift is performing for three consecutive nights in Warsaw starting Thursday.

On Aug. 1, 1944, poorly armed young city residents rose up against the German forces that had brutally occupied their nation for five years, battling them in the streets of the capital for over two months. The Soviets were approaching in their march westward against the German forces, and the Poles held out hope for help.

The Germans, with their professional army and superior weaponry, killed 200,000 Polish fighters and civilians and razed the city in revenge.

Germany in past decades has made many gestures of remorse, helping to bring about reconciliation.

Poles, however, remain bitter toward the Soviets for allowing the slaughter when they could have intervened.

Today the uprising is remembered by Poles as one of the most important moments in a long history of independence struggles against Russia and Germany.