So when will it be?

North Korea vowed last month to carry out its third nuclear test but has said nothing about timing. As a result, the building suspense in Seoul has prompted many to look at the dates Pyongyang has chosen for past atomic tests, as well as rocket and missile launches.

Dates and numbers have great symbolic importance to North Korea's government. So Pyongyang often schedules what Washington calls "provocative acts" around U.S. holidays and important South Korean political events, an effort to send none-too-subtle messages to its main enemies — Washington and Seoul. Pyongyang also uses the tests to give a nationalistic boost to its citizens, often favouring significant milestones of the state, party and ruling Kim family.

Here's a look at the "meaningful dates" North Korea has selected for past tests and launches, as well as future key dates on which Pyongyang might choose to stage its third nuclear test:

U.S. holidays and political events

Both previous nuclear tests were conducted as Americans celebrated U.S. holidays: Oct. 9, 2006, was Columbus Day; May 25, 2009, was Memorial Day.

North Korea also conducted missile tests on America's Independence Day in 2006. It launched a long-range rocket just a few months after Obama's first inauguration in 2009.

Some speculate that Pyongyang might make good on its atomic threat on the upcoming Feb. 18 federal holiday. That day is Washington's Birthday, also known as Presidents Day. South Korea's foreign minister has also suggested President Barack Obama's State of the Union address on Feb. 12.

South Korean political events

North Korea also has used inaugurations and elections in archrival South Korea to test its missiles and to launch rockets. South Korean President-elect Park Geun-hye will be inaugurated Feb. 25.

Last year's successful Dec. 12 long-range rocket launch, which was condemned by the U.N. as a cover for a banned missile test, came just a week before South Koreans went to the polls to choose a new president.

North Korea fired a short-range missile on the eve of former liberal President Roh Moo-hyun's 2003 inauguration. Days before that, a North Korean jet also intruded into South Korean airspace over the Yellow Sea, turning back as warplanes in South Korea scrambled.

And in 1998, six months after liberal South Korean President Kim Dae-jung took office, North Korea launched what it said was a rocket carrying a satellite.

North Korean milestones

A failed North Korean rocket test last year fell on April 12, days before the April 15 celebration of the centennial of the birth of national founder Kim Il Sung.

The 2006 nuclear test also came a day before the 61st anniversary of the founding of the ruling Workers' Party.

North Korea has repeatedly sought to link the legacy of late leader Kim Jong Il — Kim Il Sung's son and the father of current leader Kim Jong Un — to the development of its nuclear program. His birthday falls on Feb. 16. Two days before that, Feb. 14, is the anniversary of Kim Jong Il being posthumously named Generalissimo.