The flag of Norway is a red field with a blue cross outlined in white, the cross set nearer the hoist than the fly. It has looked essentially the same since 1821.
Meltzer's proposal
Fredrik Meltzer, a member of the Storting, put the design forward, and it was adopted on July 17, 1821. Until then Norway had used other countries' flags. Under Danish rule it flew the Danish flag; the later union with Sweden produced a combined banner drawing on both national traditions.
The timing was not accidental. Independence movements were spreading across Europe, and Meltzer's choice of red, white, and blue borrowed from the French tricolor and the American flag while the cross kept the flag legibly Nordic. A Union Badge was added in 1844 to mark the political union with Sweden, and it stayed there until Norway became fully independent in 1905, after which the flag went back to its plain form. It has not changed since.[1]
The cross and the colors
The Nordic cross is the family resemblance. Denmark, Sweden, Finland, and Iceland all use it, in different colors and proportions, and the Danish and Swedish flags in particular fed directly into Meltzer's design. What sets Norway's apart within that group is the color scheme, close enough to the French tricolor that the resemblance gets remarked on.
The red, white, and blue were understood at the time as the colors of freedom, borrowed from the French Revolution and the American flag. Whatever later readings have been attached to the blue cross, that is where the palette came from.
May 17 and everyday use
Norwegian law sets out which days the flag is flown, Constitution Day on May 17 above all, along with occasions such as the King's birthday. The general rule is sunrise to sunset, with the flag down by dusk when the day ends early. It flies at government buildings, at national events, and at diplomatic missions abroad. A few regional variants exist, mostly tied to local maritime traditions, but they are uncommon.
May 17 is where you see the flag in bulk. People wear bunads, the traditional regional costumes, and carry small flags through the parades. Norwegian painting and literature have long used the flag as shorthand for resistance and endurance, particularly in work about World War II, and it turns up at sporting events and music festivals for much the same reason.
References
[1] "Flag Protocol." Norwegian Ministry of Culture.
[2] "Norwegian Holiday Traditions and the National Flag." Encyclopedia Britannica.