Bhagavad Gita 8.24
If the sage, knowing the Supreme Spirit, goes forth with fire and light during the day, in the fortnight of the waxing moon, and in the six months before the Northern summer solstice, he will attain the Supreme.
अग्निर्ज्योतिरहः शुक्लः षण्मासा उत्तरायणम्।
तत्र प्रयाता गच्छन्ति ब्रह्म ब्रह्मविदो जनाः॥agnirjyotirahaḥ śuklaḥ ṣaṇmāsā uttarāyaṇam|
tatra prayātā gacchanti brahma brahmavido janāḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).