This day is usually celebrated as the birthday of the State Hermitage Museum. On this day (perhaps, some days earlier) 225 paintings by famous Dutch and Flemish painters were delivered to St.Petersburg. The empress Catherine bought them in Berlin. These paintings became the basis of her private collection. They were placed to secluded rooms of the royal Winter palace and for this reason became known as the Hermitage. Actually, the exact day of the founding is not known, but this day (25 November Old Style) is the day of St. Catherine, and it was thought to be a good idea to celebrate the founding of the Hermitage on the day of the saint patron of its founder.
Catherine continued to build up the collection and by 1774 there were 2,080 paintings in the Winter palace. It could have been even larger, had it not been for the wreck of Vrouw Maria.
A very good article on the history of the Hermitage museum was written by Dr. Vladimir Matveyev: Treasures of the Hermitage:
The foundation of the Hermitage is traditionally dated to 1764, when the first acquisition — a collection of 225 paintings by Western European masters — was delivered to St. Petersburg. Given her vast resources, Catherine II was able to secure most of the treasures that were offered to her, and when Frederick II of Prussia fell into financial difficulties and was unable to purchase the collection of paintings which the Berlin dealer Johann-Ernst Gotzkowsky had formed for him, Catherine bought it instead. The collection contained several masterpieces, including Frans Hals's "Portrait of a Young Man Holding a Glove".
The Hermitage collections grew rapidly. The first catalogue, published in 1774, numbered over 2,000 canvases. Apart from the acquisitions of major collections, individual paintings, many of which are now internationally-recognised masterpieces, were purchased for the Museum from private sources and at auction. Catherine II also commissioned a number of celebrated artists to provide works expressly for the Russian collection. Thus, Boucher painted his "Pygmalion and Galatea" for St Petersburg's new Academy of Fine Arts; Chardin painted his famous "Attributes of the Arts" for the Conference Hall of the Academy (although this remained in the Hermitage); and Sir Joshua Reynolds painted his allegorical picture "The Infant Hercules strangling the Serpents", which symbolized Russia's growing power.
The fine art treasures amassed by Catherine II were originally accommodated in the rooms of the Winter Palace, the main residence of the Russian tsars, which was built on the banks of the river Neva in 1754-1762 to the designs of the architect Francesco Bartolommeo Rastrelli. As the collection grew, successive buildings were added: the Small Hermitage in 1764-1775, designed by Yury Felten and Jean-Baptiste Vallin de la Mothe; the Old (Large) Hermitage in 1771-1787, also designed by Yury Felten; and the Raphael Loggia, added onto the latter building alongside the Winter Canal, its first-floor gallery being a replica of the one painted by Raphael and his pupils in the Vatican Papal Palace.
In 1852, a major development in the history of the Imperial collection was inaugurated by Nicolas I when a new museum building was opened to the public. Called the New Hermitage, it had been built in 1839-1851 to designs by the Munich architect Leo von Klenze. The portico facing what used to be Millionnaya Street was adorned with ten huge Atlantes, carved out of blocks of granite by the sculptor Alexander Terebenev. With the construction of the new buildings, an up-to-date inventory and catalogue of the Museum's collections was carried out, a task made even more necessary by the palace fire. For the first time Russian art was acknowledged with its own department, as were classical antiquities. The two galleries in the Small Hermitage, now left vacant, were to house the Romanov family portraits and the Memorial Collection of Peter I from the Kunstkamera, together with the collection of jewellery.