Short answers, pulled from the story.
The 10th of June is Portugal's national day because it is the date on which Luís de Camões died in 1580. Camões is considered Portugal's greatest poet, and his death date was chosen to mark the national day in his honor.
Os Lusíadas is a Portuguese epic poem that follows the voyage of Vasco da Gama's fleet around the Cape of Good Hope to India. The poem runs to ten cantos, 1,102 stanzas, and 8,816 decasyllabic verses written in ottava rima. It weaves classical mythology, including the gods Venus and Bacchus debating the fleet's fate, with episodes from Portuguese history.
According to tradition, Camões was shipwrecked near the mouth of the Mekong River on his return voyage toward Goa and managed to save only himself and the manuscript of Os Lusíadas. His friend and historian Diogo do Couto, who partly witnessed the poem's composition, recorded that the shipwreck profoundly reshaped the poem's themes, an effect noticeable from Canto VII onward.
King Sebastian granted Camões a pension of no more than fifteen thousand réis a year, describing him as "Luís de Camões, noble knight of my House", in payment for services rendered in India. The pension was paid irregularly and appears to have lapsed after about three years.
Torquato Tasso said Camões was the only rival he feared and dedicated a sonnet to him. Cervantes called him the singer of Western civilization. Friedrich Schlegel named him the ultimate exponent of creation in epic poetry. Voltaire, Montesquieu, Goethe, and Sir Richard Burton also wrote admiringly of his work in subsequent centuries.
Rimas was published posthumously in 1595 from manuscripts, and apocryphal poems were included from the start. Later editions, especially the 1685 edition of Faria e Sousa, systematically altered the text, so that the original 65 sonnets swelled to 352 by the 1861 Juromenha edition. Scholarly efforts to purge the canon of forgeries and corrupted readings did not begin in earnest until the late 19th century.