Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 3 · Name Letters and Chakras
युद्धजयार्णवः (चक्राणि)
शनिभागं त्यजेद्युद्धे दिनराहुं वदामि ते ।रवौ पूर्वेऽनिले मन्दे गुरौ याम्येऽनले भृगौ ॥ ८ ॥
In the full Agni Purana this is chapter 142, verse 8.
In war, avoid Saturn's share. Now I will tell you the Dina Rahu (Rahu of the weekday). On Sunday he is in the east. On Saturday, the north-west. On Thursday, the south. On Friday, the south-east.
Simple meaning: In war, avoid Saturn's share. Rahu sits in a different direction each weekday: east on Sunday.
अग्नौ कुजे भवेत्सौम्ये स्थिते राहुर्बुधे सदा ।
Rahu's direction changes each weekday. Serpent Rahu moves round the corners, three hours in each.
वायौ संवेष्टयित्वा च शत्रुं हन्तीशसंमुखम् ।
Serpent Rahu circles and kills the enemy. At full moon, Tithi Rahu is in the south-east.
अमावास्यां वायवे च राहुः संमुखशत्रुहा ।
At the new moon Rahu is in the north-west. The letters are split into groups by direction.