Princess Charlotte first attended the royal occasion in 2016 when she was 13 months old (which is a similar age to Louis), and she delighted crowds with her mini royal wave.
The ceremony is attended by all senior royals, and also marks the official commemoration of the Queen's birthday.
The extravagant event involves a military parade involving thousands of officers, horses and musicians.
In the past, royal family members including Prince William, Prince Charles and Princess Anne have participated on horseback.
On the day we can expect to see the royal family travel in horse-drawn carriages and arrive at Buckingham Palace, where they greet crowds and watch the RAF flypast from the Palace balcony.