Nine of the accused are charged with rebellion for their role in the dramatic attempt to break from Spain in October 2017.
They face up to 25 years in jail.
The source, who asked not to be named, confirmed the date of the trial but gave no further details.
In pre-trial detention for months, the nine were being transferred from the northeastern region of Catalonia to prisons near Madrid on Friday.
Supporters in Barcelona cheered them as they were taken away, waving the region's red, yellow and blue separatist flag.
Three others will also stand trial, accused of disobedience and misuse of public funds.
Supporters of independence for Catalonia call the trial a “farce”.
But many Spaniards support it, having looked on in disbelief as the rich region's then executive tried to secede.
Catalonia's president at the time of the secession bid, Carles Puigdemont, is also accused of rebellion.
He fled Spain after the region's majority-separatist parliament made a short-lived declaration of independence on October 27, 2017.
He is not among the 12 on trial in Spain, which does not try suspects in absentia.
The trial's main protagonist is former regional vice president Oriol Junqueras, who opted to remain in Spain. He faces up to 25 years in jail on charges of rebellion and misuse of public funds.
READ MORE: Supreme Court will judge 12 Catalan separatists while six others to face trial in Catalonia