Willem-Alexander becomes the first King of the Netherlands in more than 120 years on Tuesday when Queen Beatrix passes the crown to her eldest son.
Thousands of well-wishers gathered in Amsterdam's Dam Square early on Tuesday, waiting to see the new King and Queen when they step out on the balcony of the Royal Palace.
"He's ready, in every way," Queen Beatrix said of her 46-year-old son, a water management specialist who is expected to bring a less formal touch to the monarchy, as she bid farewell to the nation in a subdued televised address on Monday night.
April 30, or Queen's day, is always a day for partying in the Netherlands. Many people took Monday off work and started celebrating in earnest from Monday evening with street bands and music.
But this year the city of Amsterdam is putting on a special show to celebrate the investiture of Willem-Alexander and the abdication of Beatrix, 75, who wants to retire after 33 years in the job.
Amsterdam was awash with orange, the royal colour. Houses were covered in bunting and flags, shop windows were stuffed with orange cakes, sweets, clothes and flowers and many partygoers were decked out in the royal colours.
Nearly a million people were expected to join the street party with dancing to bands and DJs, helping create a carnival atmosphere.
"He (Willem-Alexander) knows what is needed. He unites people. He has made it possible for the different generations to mingle more," said 40-year old Margriet Dantuma, dressed in an orange skirt, as she joined others on the Amsterdam pavements putting out impromptu stalls of bric a brac for sale.
The royals are broadly popular, with 78 percent of Dutch in favour of the monarchy up from 74 percent a year ago, according to an Ipsos poll.
But they have been stripped of their political influence, and no longer appoint the mediator who conducts exploratory talks when forming government coalitions.
RARE TRIP BY JAPAN'S PRINCESS
Britain's Prince Charles and Japan's Crown Princess Masako, who is making her first foreign trip since falling ill a decade ago, will be among 2,000 visitors at the official ceremony.
"There will be tears on Tuesday," said Dutch Prime Minister Mark Rutte, paying tribute to "this formidable lady who has ruled this country for over 30 years".
Beatrix will sign her abdication at 10:00 a.m. (0800 GMT), at which point Willem-Alexander immediately becomes king and his wife Maxima, a popular former investment banker from Argentina, becomes queen.
All three will appear on the palace balcony to wave and address the crowds in Dam Square.
They will then head from the palace to the 600-year-old Nieuwe Kerk, or New Church, next door where the king will swear an oath to uphold the Dutch constitution before lawmakers.
The Dutch monarch is never crowned, since, in the absence of a state church, there is no cleric available to carry out the coronation. But there is a crown, which will sit on a table next to him throughout the ceremony, along with other regalia that constitute the crown jewels.
Willem-Alexander will wear a royal mantle that has been used for investitures since 1815, although it has been repaired and altered at least twice over the past century, for the investitures of his mother and grandmother.
Celebrations are expected to continue through the evening with a water pageant along the IJ, Amsterdam's historic waterfront.