Ukraine's lawmakers are set to ratify a landmark EU association agreement - in proceedings simultaneous with a session of the European Parliament.
Both sides will start the procedure at 10:00 GMT, in what Ukraine's president said would be a "historic" day.
But the implementation of a free trade deal - part of the package - has been postponed until 2016, apparently under pressure from Russia.
This comes as a fragile ceasefire largely holds in eastern Ukraine.
There were reports of shelling around the government-held airport of Donetsk and other towns.
Pro-Russian rebels have been engaged in heavy fighting with government forces in the Donetsk and Luhansk regions since April - a month after Russia annexed southern Crimea peninsula.
Some 3,000 people have been killed over the past five months of conflict.
Russia denies sending troops and heavy weaponry to help the rebels, as alleged by Ukraine and the West.
Poroshenko - who initialled the deal back in June - said that "de facto this is a reform programme in our country" aimed at guaranteeing "the rule of law, freedom of speech and decisive anti-corruption steps".
The move is seen by many as a culmination of the country's recent deadly political upheavals, bringing the former Soviet republic closer to the EU and away from Russia's sphere of influence.
In November, hundreds of thousands of Ukrainians took to the streets to protest against then President Viktor Yanukovych's abrupt decision to cancel the agreement's original planned signing.
However, Mr Poroshenko is facing growing criticism in Ukraine for apparently caving in to pressure from Moscow and postponing the implementation of the free trade deal with the 28-member bloc.
The implementation of the deal - part of the broader agreement - had been originally set for November, but last week was delayed until the end of 2015.
Despite this, the EU said Ukraine would continue enjoying unilateral tariff-free trade with the bloc until then.
Russia has repeatedly warned that once the deal comes into effect, European exports could find their way into Russia duty-free, since Kiev has its own beneficial trade conditions with Moscow. Or less expensive European goods could force Ukrainian goods out of their own market and into Russia.
The Kremlin has also warned it would be forced to introduce protective measures, hurting Ukraine's struggling economy.
Some politicians in Ukraine reacted with dismay to the postponement.
"I am speechless," Dnipropetrovsk Region Deputy Governor Svyatoslav Oliynyk was quoted as saying by the AFP news agency.
"The last time this happened, we had EuroMaidan," he added, referring to the mass protests in November.
(BBC)
