The father-to-be joined his siblings Pierre and Charlotte, as well as his mother, grandmother-to-be Princess Caroline, at a mass which took place at Saint Nicholas Cathedral in Monte Carlo.
Prince Albert was also in attendance with his glamorous wife Princess Charlene, who was elegant as ever in a smart navy blue coat and a black hat with netting attached. Princess Stephanie completed the family get together.
National Day is one of the highlights of the Monaco social calendar. Although the House of Grimaldi has ruled Monaco since 1297, the November 19 holiday celebrates the date when the state's sovereignty was officially recognized by the Franco-Monegasque Treaty of 1861.
While significant others of the younger Monagasque royals don't usually attend the celebrations, there was speculation Tatiana might make an appearance in her official capacity as Andrea's fiancée.
Andrea's mum Caroline confirmed news of a wedding in July, in an announcement via her press office simply saying: "I am happy to announce the engagement of my son, Andrea Casiraghi to Miss Tatiana Santo Domingo." No further details about the nuptials have been disclosed. But given the impressive pedigree shared by the Monegasque golden boy and his society beauty, glamour and high-profile guests will certainly feature – as was the case when Albert wed Charlene last year.
Until such time as his uncle has an heir Andrea is also second in line to the throne, so there will be considerable fanfare.
Not long after the wedding announcement came reports Tatiana may have been expecting. When Andrea and his fiancée touched down on dry land following a few days cruising the Cote d'Azur on Pacha III, the Grimaldi's family yacht, pictures emerged of Tatiana with a curvier figure.
But since the dress she wore was loose and flowing, and with no confirmation from the palace, commentators were unsure. Tatiana, the granddaughter of the late Colombian mega-mogul Julio Santo Domingo, worth $6bn, is known for her love of bohemian clothing.
In fact, she often uses her twitter feed to show off loose peasant style garments that she has picked up from artisans around the world, for her business Muzungu Sisters.
But then the happy news was confirmed after the pretty brunette attended a fashion awards ceremony in Madrid, showing off an undeniable baby bump. The heiress confirmed she is expecting, but gave no further details about the pregnancy. It's thought she is in her second trimester and the baby will be born early next year. The new arrival will be third-in-line to the Grimaldi throne.
The laidback lovebirds probably don't have their minds on the succession, however, they will be too busy enjoying this special period before becoming husband and wife - and first-time parents - in 2013.