Police were called to Pryzm, Lower Parliament Street, Nottingham city centre, where the man is believed to have been surrounded by a group and punched.
The man was taken to hospital with a head injury and continues to recover.
Police were quickly at the scene but found a number of suspects had dispersed.
Enquiries, including reviewing CCTV, have been ongoing since the incident, which happened at around 1.25am on Sunday 9 January 2022.
Officers have now released images of five men they would like to speak to as the investigation continues.
Detective Constable Sean Davison, of Nottinghamshire Police, said: “A man was left needing to go to hospital and with injuries to his head, which could have been devastating, and it is extremely fortunate he is now recovering.
“The behaviour reported is unacceptable and we are working hard to understand the circumstances of what happened. The man was simply enjoying a night out and this is not how anyone should expect their evening to end.
“We have now released images of five men we would like to speak to. We believe they may have important information which could help our enquiries.
“We are determined to find those responsible and would ask anyone who recognises these men, or anyone who witnessed the incident or has further information, to get in touch with Nottinghamshire Police on 101, quoting incident 55 of 9 January 2022.”
Anyone with information can also report this anonymously through the independent charity Crimestoppers on 0800 555 111.