TORONTO -- The Queen will be surrounded by family on Monday – almost everyone and everything that is important to her: The Commonwealth Family of Nations and the Royal Family.

Commonwealth Day is always a highlight in The Queen's annual diary – a celebratory opportunity for Her Majesty as Head of Commonwealth to interact with citizens of the organization's 54 countries, with a new one added just last month– the Republic of Maldives.

In recent times, the Monarch has been joined by many members of her family at this celebration, including the Prince of Wales who will succeed Her Majesty as Head of the Commonwealth in due course, as decided last year.

But in 2020, Commonwealth Day takes on added poignancy as it will be the last official royal engagement by the Duke and Duchess of Sussex. All eyes will be on them as their relationship with the Royal Family is placed under scrutiny in public, for the first time since January.

For the Commonwealth itself, the Sussex participation on Monday will be bittersweet. The Duke and Duchess were expected to play a major role in that organization. Prince Harry and Meghan accepted roles as president and vice-president of the Commonwealth Youth Trust and Meghan as Patron of the Association of Commonwealth Universities. These seemed like perfect tasks for the Duke and Duchess given their youth and vitality.

Their appeal to a new generation is just what the Commonwealth needs. There was much excitement and hope about what they might do and how they would engage.

While initial indications are that they will try and continue, Prince Harry will not be able to fulfill a third role as Commonwealth Youth Ambassador, given that this would require government support for travel. It also remains to be seen who will organize Sussex engagement with the Youth Trust and the Commonwealth Universities.

And who knows what priority this will receive where Prince Harry and Meghan may well place their private interests ahead of these patronage commitments.

The Commonwealth could have used the boost from the Sussexes as it is presently facing very big challenges - a leadership and existential crisis. A financial scandal has caused the U.K., Australia and New Zealand to withhold funding and there is a continued drift in its mission and what it stands for. The future of the Secretary General, Baroness Scotland, is also an issue.

The next Commonwealth Heads of Government Meeting (CHOGM) is later this year in Rwanda and given concerns over human rights issues in that country, it may well be a difficult meeting and there could be boycotts. For many, the Commonwealth is only visible at its quadrennial Games. It is searching for a role and reason to exist and Canada plays a key role in all of this as the senior member outside the U.K.

On Monday, The Queen will be happy to be surrounded by her two families, but it will be nostalgic given what might have been and with the storm clouds circling around in both families.

Keeping the Commonwealth going has been one of The Queen’s key accomplishments in her life, but without the energy that Prince Harry and Meghan bring as well as the brewing troubles on the horizon, it may be one of the most bittersweet Commonwealth Days in Her Majesty’s reign.