The hearing for the nurses and a Palestinian doctor had been set for 31 January, but Dimitar Tsanchev, spokesman for the Bulgarian Foreign Ministry, told reporters that date had been moved forward.
He said the Bulgarian embassy in Tripoli had been informed of the move by the nurses' lawyer.
The nurses have been imprisoned since 1999. They face death by firing squad after being found guilty of infecting 426 children with HIV in a hospital in the Mediterranean port of Benghazi. About 50 of the children have since died.
The nurses say their confessions were extracted under torture. Aids experts also told a Libyan court the outbreak started before the nurses arrived and was probably caused by poor hygiene.
Bulgaria, the European Union and the US have rejected the guilty verdicts as unfair. The criticism hampers recent efforts by the Libyan president, General Muammar al-Qadhafi, to end his diplomatic isolation.
Libya has suggested the death sentences could be quashed if the children receive ample humanitarian aid.