Prince Louis will be christened in front of close members of the royal family on Monday.
The 11-week-old, who is fifth in line to the throne, will be baptised by the Archbishop of Canterbury, the Most Reverend Justin Welby, in the Chapel Royal at St James' Palace.
Prince George, four, and three-year-old Princess Charlotte will be there for their younger brother's big day.
It will be the first time Prince William, Kate and their three children will be seen together as a family of five.
Kensington Palace has yet to release the guest list, but royal christenings are usually a small, family affair.
Louis' godparents, who will be announced on the day, will take part in the private 40-minute traditional religious ceremony, which begins at 4pm.
There is speculation the Cambridges, who usually opt for close friends and confidantes, may choose William's trusted private secretary Miguel Head, who is leaving the post this month, to be a godfather.
The baby prince will be dressed in the same frilly, cream gown used for George and Charlotte's baptism.
The robe is a replica of the intricate lace and satin dress made for Queen Victoria's eldest daughter in 1841.
