In 1612, Marie de' Medici*, widow of Henry IV of France, purchased houses there from the Duke of Piney-Luxembourg to build a sumptuous palace to the designs of French architect Salomon de Brosse. She wanted to call it the Medici Palace, but the Parisians - who hated her - kept using the name "Luxembourg".
* Yes, she's one of THOSE Medicis - but a hundred years after Ezio knew them...
In 1750, the Luxembourg became the first museum in Paris, where visitors could view paintings from the Royal collection.
In 1793, the Luxembourg Palace was briefly converted into the "Maison Nationale de Sûreté", or national prison. Danton was detained there before being sent to the guillotine, having done the same himself for so many of his enemies.**
** Ouch, nice little dig there. Of course, you wouldn't expect an Abstergo historian to be sympathetic to an Assassin sympathiser, would you? Also, I think it's fair to say that Danton kind of deserves it...
In 1801, Bonaparte offered the building to the French Senate, the function it still serves today. Transformations overseen by architect Alphonse de Gisors in 1836 modified the overall structure, to the extent that Marie de' Medici would probably not have recognized her former abode. The garden, however, is a favorite among Parisians.