Which Calendar did the Jews follow when the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, ordered the Muslims to fast ‘Aashooraa’ (the tenth of Muharram)?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam,  is His slave and Messenger.

The Jews used to depend on the solar months which are calculated according to the movement of the sun. Ibn Hajar said,

His saying “when the Prophet, sallallaahu ‘alayhi wa sallam, reached Al-Madeenah, he found the Jews fasting ‘Aashooraa’ (tenth of Muharram)” was a sort of confusion because he reached in Rabee‘ Al-Awwal. This confusion was answered that perhaps his knowledge of this matter took place later in the second year. Some late scholars said, “It is probable that their fasting took place according to the solar months, then, it is possible that ‘Aashooraa’ would take place in Rabee‘ Al-Awwal and there will be no confusion.”

This opinion was adopted by Ibn Al-Qayyim in Zaad Al-Ma‘aad fi Hady Khayr Al-‘Ibaad, when he said, “Fasting of the people of the Book is observed according to the solar calendar.”

Allaah Knows best.