By Agence France-Presse
The ceremony at St. George's Chapel in Windsor Castle will be a traditional Church of England service, but US TV star Markle has ensured that her marriage into British royalty will have a strong American flavour.
After a firebrand US preacher revs up the sermon, The Kingdom Choir will sing "Stand By Me", the stirring, soulful tune released by Ben E. King in 1961, that topped the charts in both the United States and Britain.
And the service will have further touching moments for Harry, being supported by his best man, brother Prince William.
The last hymn before the newlyweds leave will be "Guide me, O thou great redeemer" -- the last hymn sung at their mother Diana, princess of Wales's funeral in 1997.
The bride and groom have put their own modern twist on their big day.
The music, the springtime flowers and lack of political bigwigs show they have set the tone and feel of the event, within the framework of a classic Anglican wedding ceremony.
Printed before Markle's father Thomas definitively pulled out on Thursday having undergone heart surgery, the order of service says he will walk her down the aisle.
Harry's father Prince Charles, the Prince of Wales, will fill in.
'Stand By Me'
David Conner, the Dean of Windsor who is the chapel's senior cleric, will give the introduction.
"The gift of marriage brings husband and wife together in the delight and tenderness of sexual union and joyful commitment," he was to say.
Conner will ask "anyone present who knows a reason why these persons may not lawfully marry, to declare it now".
Assuming no objections, Justin Welby, the Archbishop of Canterbury who is the Church of England's spiritual leader, will oversee the declarations.
The couple will pledge to love, comfort, honour and protect each other, both using the answer "I will".
Diana's sister Jane Fellowes will read from the Song of Solomon, before Michael Curry, the head of the Anglican church in the United States, gives the ceremony's main speech.
A lively, animated speaker in the US tradition, the primate of the Episcopal Church will deliver his own address on the nature and duties of marriage.
The church's first African-American leader, Curry could be the star turn in a setting more used to rigid formality.
He is immediately followed by "Stand By Me", the prelude to the couple exchanging their vows.
The couple have chosen vows using modern rather than traditional language, meaning they will say "you" instead of "thee" and "thou".
They have perhaps noted how several previous brides have muddled up the royals' multiple given names, so Markle will refer to the groom as simply Harry rather than Henry Charles Albert David.
Till death us do part
And like recent royal brides, Markle will not vow to "obey" her husband.
They will pledge to take each other "to have and to hold from this day forward; for better, for worse, for richer, for poorer, in sickness and in health, to love and to cherish, till death us do part."
Welby then declares them legally wed, saying: "I therefore proclaim that they are husband and wife."
The prayers will be read by the Jamaican-born Reverend Rose Hudson-Wilkin, Queen Elizabeth II's chaplain, and Archbishop Anba Angaelos, the head of the Coptic church in Britain.
"Guide me, O thou great redeemer", the final hymn, was also the first hymn sung at the 2011 wedding of William and his wife Kate.
The song is effectively a Welsh national hymn and is often sung at rugby internationals. It will be seen as a nod to Harry's 33 years as, formally, Prince Henry of Wales.
The couple then go to sign the marriage register.
When they return, the service will end with the national anthem, "God Save the Queen".
Harry will bow to Queen Elizabeth and his bride will curtsey as they leave, stepping out onto the church's West Steps to greet the world as husband and wife.