The exchange was brokered by the Russian and Ukrainian presidents at peace talks in Paris earlier this month.
Zelenskiy and Russian President Vladimir Putin in their first meeting failed to find a compromise to end the five-year war that has killed 14,000 people, but agreed to revive the peace process and exchange prisoners.
“The exchange should be tomorrow. We're all waiting for it,” Zelenskiy said Saturday, calling it “the most difficult task this year.”
Separatist officials confirmed the exchange was to take place Sunday and said they expected to swap 55 Ukrainians for 87 rebels.
Earlier this week Ukrainian media reported that a court in Odessa released 11 people accused of terrorism in preparation for the exchange. Four others were released from jail in Kharkiv on Saturday.
The last major prisoner swap between separatist rebels and Ukrainian forces took place in December 2017, with 233 rebels exchanged for 73 Ukrainians.