The British monarch was released March 4 after a brief hospital stay, and palace officials had said Monday's Abbey event would be her first public appearance since she became ill.
Royal sources were careful to emphasize the Queen's health was no worse Monday, but that physicians had told her to "pace herself" at the "tail end" of her illness.
"When you're 86 it takes longer to get over gastroenteritis than when you're younger," the source told the Telegraph. "Otherwise the Queen is in good health and hopes to carry out some of her planned engagements this week."
The Queen is still expected to attend her evening events, including a gathering with Commonwealth leaders at which she will sign a major charter declaring equality for women and gay people.
Monday night's reception has the added advantage of possible inconspicuous exit should the Queen not feel well, unlike the Westminster Abbey service.
The Commonwealth Charter, considered a watershed statement on gender and sexual equality from the monarchy, has been hailed as a "21st Century Magna Carta." It states: "We are implacably opposed to all forms of discrimination, whether rooted in gender, race, colour, creed, political belief or other grounds."
In pre-recorded remarks for broadcast, the Queen will emphasize those sentiments of equality:
"Ambition and curiosity open new avenues of opportunity.
That is what lies at the heart of our Commonwealth approach: individuals and communities finding ways to strive together to create a better future that is beneficial for all.
Our shared values of peace, democracy, development, justice and human rights - which are found in our new Commonwealth charter - mean that we place special emphasis on including everyone in this goal, especially those who are vulnerable."
The Charter also endorses landmark legislation which will allow a firstborn child, regardless of sex, to be in direct succession to the British throne. This could have immediate impact if the first child of Catherine, Duchess of Cambridge, and Prince William, due this summer, is a daughter.