Complex sentence: The Muslim scholar Abū Rayḥān al-Bīrūnī wrote that the month Dey is also called "khor", with first day of the month called "khorerooz".
Split into simpler sentences: The Muslim scholar Abū Rayḥān al-Bīrūnī wrote that the month Dey is also called "khor". The first day of the month is called "khorerooz".