Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
monthly [ˈmʌnθlɪ] adj ежеме́сячный; (ticket) ме́сячный
adv ежеме́сячно
twice monthly два́жды в ме́сяц
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.