Delegates from around the North have been arriving in Pyongyang ahead of the assembly session. They visited statues of previous leaders Kim Il Sung and his son, Kim Jong Il, state media reported.
North Korea is emerging as one of the most pressing foreign policy problems facing the administration of U.S. President Donald Trump. It has conducted five nuclear tests, two of them last year, and is working to develop nuclear-tipped missiles that can reach the United States.
The Trump administration is reviewing its policy towards North Korea and has said all options are on the table, including military strikes, but U.S. officials said non-military action appears to be at the top of the list if any action were to be taken.
The U.S. Navy strike group Carl Vinson was diverted from planned port calls to Australia and would move toward the western Pacific Ocean near the Korean peninsula as a show of force, a U.S. official told Reuters over the weekend.
U.S. officials said it would still take the strike group more than a week to arrive near the Korean peninsula.
Trump and his Chinese counterpart, Xi Jinping, met in Florida last week and Trump pressed Xi to do more to curb North Korea's nuclear program.
China and South Korea agreed on Monday to impose tougher sanctions on North Korea if it carried out nuclear or long-range missile tests, a senior official in Seoul said.
On Tuesday, a fleet of North Korean cargo ships was heading home to the port of Nampo, the majority of it fully laden, after China ordered its trading companies to return coal from the isolated state to curb coal traffic, sources with direct knowledge said.
The order was given on April 7, just as the U.S. and Chinese leaders were set for the summit where the two agreed the North Korean nuclear advances had reached a "very serious stage," U.S. Secretary of State Rex Tillerson said.
Following repeated missile tests that drew international criticism, China banned all imports of North Korean coal on Feb. 26, cutting off the country's most important export product.
As well as the anniversary of Kim Il Sung's birth, there are several other North Korean anniversaries in April that could be opportunities for weapon tests, South Korean officials have said.
The North is seen ready to conduct its sixth nuclear test at any time, with movements detected by satellites at its Punggye-ri nuclear test site.