The opposition says it has more than 172 votes in the 342-seat assembly, which needs a quarter of members present for a quorum.
Saturday's vote will cap a dramatic week during which Khan sidestepped an initial no-confidence vote before getting the loyalist president to dissolve parliament and call fresh elections.
But the Supreme Court Thursday ruled all his actions illegal, and said the national assembly -- where Khan has lost his majority -- must decide his fate.
The court's judgement was broader than expected after the chief justice said earlier this week the bench would only rule on the legality of the initial no-confidence motion being blocked.
End of 'doctrine of necessity'
Constitutionalists praised the verdict, calling it an end to the so-called "doctrine of necessity" that has seen courts throughout Pakistan's history rule against clear illegality, but accept the consequences as being good for the country.
The decision was met with jubilation by the Pakistan Muslim League-N (PML-N) and the Pakistan Peoples Party (PPP), two normally feuding groups that combined to oust Khan.
PML-N leader Shehbaz Sharif, brother of three-time prime minister Nawaz Sharif and likely to replace Khan, said the decision "has saved Pakistan and the constitution".
"Democracy is the best revenge", tweeted PPP leader Bilawal Zardari Bhutto, the scion of another political dynasty. His parents are assassinated former prime minister Benazir Bhutto and ex-president Asif Ali Zardari.
How long the next government lasts is also a matter of speculation.
The opposition said previously they wanted an early election -- which must be called by October next year -- but taking power gives them the opportunity to set their own agenda and end a string of probes they said Khan launched vindictively against them.
It could also pave the way for a comeback by Nawaz Sharif, who has not returned from Britain since being allowed to leave jail in 2019 to seek medical treatment abroad.
He was barred by the Supreme Court from holding public office after graft revelations, and sentenced to 10 years in prison by an accountability court.
There had been high hopes for Khan when he was elected in 2018 on a promise of sweeping away decades of entrenched corruption and cronyism, but he struggled to maintain support with soaring inflation, a feeble rupee and crippling debt.
There has also been a rise in violence by Islamic militants encouraged by the return to power of the Taliban in neighbouring Afghanistan.
Underlying issues remain
Political analyst Hasan Askari told AFP any new government will still have to deal with the underlying issues.
"Conflict and confrontation will persist... the prospects of political harmony and long-term stability are minimal," he said.
Pakistan has been wracked by political crises for much of its 75-year existence, and no prime minister has ever seen out a full term.
Khan has blown anti-US sentiment into the political atmosphere by saying the opposition had colluded with Washington to oust him.
The 67-year-old says Western powers wanted him removed because he will not stand with them against Russia and China.
Publicly the military appears to be keeping out of the current fray, but there have been four coups since independence in 1947 and the country has spent more than three decades under army rule.