Manchester City Council announced on Monday that the free event will be held in Albert Square and across the region to remember the 22 killed in the atrocity at the Ariana Grande concert.
The May 22 event, titled Manchester Together - With One Voice, will be a "united act of remembrance", the council said.
Officials are seeking choirs, community groups and schools to sing songs that "evoke pride and solidarity".
The BBC said the final 30 minutes of the sing-along will be broadcast on its Radio Manchester.
"Coming together in song will once again demonstrate that remarkable sense of togetherness," council leader Sir Richard Leese said, according to the broadcaster.
"It is a fitting way for us to honour those who went full of joy to a music concert but never came back, or saw their lives changed forever."
In a separate event, the council has called for suggestions of lyrics to be projected on pavements and buildings in St Ann's Square between May 22-26.
A civic memorial service attended by families of the victims will also be held at Manchester Cathedral.
May 22 will mark one year since suicide bomber Salman Abedi detonated a home-made device in the foyer of the venue as 14,000 people left the Grande concert.