The Queen said the young were critical to the organisation's future
The Queen will mark the 60th anniversary of the creation of the Commonwealth when she attends a special service at Westminster Abbey.
High Commissioners from the 53 Commonwealth nations are also expected to take part in the celebration.
But a poll of UK adults has found its importance to Britons is in decline.
While the YouGov survey suggests many feel warmly towards the Commonwealth, just 35% of 18 to 34-year-olds would be sorry if the UK withdrew.
The research, commissioned by the Royal Commonwealth Society (RCS), questioned 2,119 UK adults about their attitudes towards the institution.
Dr Danny Sriskandarajah of the RCS said the 53-member grouping needed to keep "reaching out".
The Queen said the Commonwealth must keep its focus on the young.
In her Commonwealth Day message, she said that it had evolved out of all recognition, representing the rich diversity of humankind but representing shared values of democracy, freedom and human rights.
"Nearly one billion people of today's Commonwealth are under 25 years of age," she said.
"These are the people that this association must continue to serve in the future."
'Continue to serve'
From eight countries to a third of the world's population, the Commonwealth has come a long way since its launch in 1949.
But the survey suggests that the UK public's level of knowledge about it is poor.
Just one in five respondents were able to name any activities undertaken by the Commonwealth, and among those the overwhelming majority identified the Commonwealth Games.
Asked which international bodies were valuable to the UK, just 33% opted for the Commonwealth, compared to 44% for NATO, 42% for the UN, 37% for the EU and 35% for the G8.
Although 42% said they would be unhappy about the UK withdrawing from the Commonwealth - compared to 4% who would be happy - some 55% said they would not mind or did not know.
And although 53% of over-55s said they would be appalled or sorry if the UK withdrew, the number fell sharply among younger respondents.
The 18 to 34-year-olds also had more trouble correctly identifying members of the Commonwealth, where its headquarters were based and who was its head.
Some 8% of 18 to 34-year-olds thought the US was a member and 3% believed President Barack Obama was head. Bono, the singer of Irish rock group U2, was believed to be the head by 1%.
However, some 70% of all respondents were able to correctly identify the Queen as the head of the Commonwealth.
Dr Sriskandarajah said it needed to connect with younger generations if it were to survive another 60 years.
"The Commonwealth has achieved much in the last 60 years, but beyond sports and the Sovereign, British people appear to know little about what the Commonwealth is or does," he added.
"Younger Brits don't seem to know or care much about the 53-member grouping."