The court said in a statement defence pleas and the defendants' last declarations had been scheduled for June 10 and 11, marking the close of a trial that will have lasted four months.
The verdict is expected in the autumn, possibly in October, according to Spanish media.
The Madrid-based court is trying the 12 in connection with a banned independence referendum held October 1, 2017 that was followed by a short-lived declaration of independence which sparked Spain's worst political crisis in decades.
Catalonia's former vice-president Oriol Junqueras faces up to 25 years in jail, the heaviest sentence, for rebellion and misuse of public funds.
He is currently in custody as the highly sensitive trial continues, as are eight other defendants.
For Catalonia's pro-independence movement, they are "political prisoners" but the judiciary and government categorically refute this.
Four of them were elected MPs in an April general election.
They were let out of custody to swear allegiance to the constitution in parliament, before being suspended on Friday.
Another defendant who was elected to the Senate has not yet been suspended.
Junqueras was also elected as a Member of the European Parliament in Sunday's vote.