The flag of the Philippines is blue over red with a white triangle at the hoist, and it is the only national flag that changes meaning when you turn it upside down.
Aguinaldo's design
Emilio Aguinaldo, who led the revolutionary movement, worked out the design while in exile in Hong Kong in 1897. It was unfurled on June 12, 1898, at the declaration of independence from Spain in Kawit, Cavite, the founding moment of the First Philippine Republic.
The flag was banned twice in the century that followed: during the American colonial period, and again under Japanese occupation in World War II. It was reinstated afterward with minor adjustments. The Flag and Heraldic Code of the Philippines, adopted on February 12, 1998, set out the official design along with the rules for use, display, and handling, and the penalties for breaking them.
Sun, stars, and triangle
The white equilateral triangle at the hoist stands for equality and fraternity. Inside it, a golden sun with eight rays represents the first eight provinces that revolted against Spanish rule. A five-pointed star sits at each corner of the triangle for Luzon, the Visayas, and Mindanao.
Blue is peace, truth, and justice. Red is patriotism and valor. White is purity and equality.
The flag inverted
In peacetime the blue band goes on top. In wartime the flag is flown with the red band on top instead. The inversion is written into protocol, and no other country's flag works this way.
The design has cousins among other revolutionary flags of the late 19th and early 20th centuries, the Cuban flag among them, which came out of a comparable independence struggle.
Where it flies
The flag appears at state functions, on public holidays, and at national events, and schools teach its history as part of the curriculum. Variants have been produced for particular occasions, including the Centennial flag used in the 1998 celebrations marking a hundred years since the first unfurling.
References
[1] Official Gazette of the Republic of the Philippines (link)
[2] National Historical Commission of the Philippines (link)
[3] Vexillological Association of the Philippines (link)
[4] "Philippine History and Government" by Gregorio F. Zaide and Sonia M. Zaide
[5] "Flags and Heraldic Code of the Philippines" (Republic Act No. 8491)