Almost 15 million people were left waiting for delayed post. Some 54 percent reported going at least a week without letters.
Three percent said they could not pay a bill, eight percent failed to receive an important document and seven percent missed a health appointment.
Citizens Advice monitoring showed disruption peaked on January 12-13 when Royal Mail admitted 77 areas suffered problems due to Covid-19.
The impact on deliveries meant 2.4 million people in England and Wales were affected each day.
Around a third of homes across London, the South-east and the North-west were most likely to experience a delay.
The charity said the situation seems to have improved but noted it is the second year running it has flagged severe Christmas disruption. Some 16.5 million people were affected last year.
It wants watchdog Ofcom to investigate Royal Mail’s performance and consider enforcement action and fines.
Dame Clare Moriarty, Citizens Advice CEO, said: “We understand the strain the pandemic has placed on Royal Mail. But after two years of disruption, it cannot let poor service become the new normal.”
A Royal Mail spokesman apologised for delays. They added: “The rise of Omicron had a particular impact during Christmas and January, with 15,000 staff off work due to sickness and self-isolation at one stage. The situation has significantly improved. The number of offices reported as being most impacted by service issues has reduced from 77 to just one today.”
A spokesman for Ofcom said: “We have made it clear to the company that it must improve as the impacts of the pandemic subside.”