Why April 15th is the speculated date of North Korea's next nuclear test

— -- So far this year, North Korea has conducted four missile launches, including as recently as last Tuesday when North Korea fired a ballistic missile into the Sea of Japan.

The missile launches have led to heightened tension on the Korean peninsula and strained relations between the U.S. and North Korea.

Now, U.S. officials are speculating that North Korea will conduct its sixth nuclear test on the country’s most important holiday - April 15th or “Day of the Sun” as it’s known in North Korea. The country has previously used holidays and anniversaries as a showcase for its arsenal of weapons.

The most recent North Korean nuclear test occurred on September 9, a holiday called the Day of the Foundation of the Republic that commemorates the day the Democratic People's Republic of Korea (DPRK) was founded in 1948, akin to the Fourth of July.

The Day of the Sun is the North Korean celebration of the birthday of its founder Kim Il Sung, who died in 1994 but is referred to as the "Eternal President."

Celebrations in North Korea traditionally include singing and dancing, fireworks, an international marathon, a military parade and a display of military strength and weapon capabilities. Festivities every five and ten years are traditionally more pronounced than others, according to KBS, the South Korean public broadcaster. 2017 marks the 105th anniversary of Kim Il Sung's birth.

Last week, the U.S. deployed the USS Carl Vinson strike group to the Sea of Japan in response to North Korea’s “pattern of provocative behavior,” national security adviser H.R. McMaster said. The strike group, however, currently remains off the coast of northwest Australia participating in exercises with the Australian Navy.

“We will hold the U.S. wholly accountable for the catastrophic consequences to be entailed by its outrageous actions,” a DPRK foreign ministry spokesman said in response.

North Korea also pointed to the U.S. military’s airstrikes in Syria last week as vindication; "The reality of today proves our decision to strengthen our military power to stand against force with force was the right choice a million times over,” a spokesperson for North Korea’s foreign minister said, according to Reuters.

In an interview with the Associated Press, North Korea's vice foreign minister said a nuclear test would occur “at a time and at a place where the headquarters deems necessary.”

"It is possible the North may wage greater provocations such as a nuclear test timed with various anniversaries..." South Korean acting president Hwang Kyo-ahn said of North Korea, Reuters reported.

North Korea is technically still at war with South Korea and its ally the U.S., since the Korean War ended in a truce instead of a peace treaty.