SB 10.22.1

śrī-śuka uvāca
hemante prathame māsi
 nanda-vraja-kumārikāḥ
cerur haviṣyaṁ bhuñjānāḥ
 kātyāyany-arcana-vratam
Synonyms: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; hemante — during the winter; prathame — in the first; māsi — month; nanda-vraja — of the cowherd village of Nanda Mahārāja; kumārikāḥ — the unmarried young girls; ceruḥ — performed; haviṣyam — unseasoned khichrī; bhuñjānāḥ — subsisting on; kātyāyanī — of the goddess Kātyāyanī; arcana-vratam — the vow of worship.
Translation: 
Śukadeva Gosvāmī said: During the first month of the winter season, the young unmarried girls of Gokula observed the vow of worshiping goddess Kātyāyanī. For the entire month they ate only unspiced khichrī.
Purport: 

The word hemante refers to the month of Mārgaśīrṣa — from approximately the middle of November to the middle of December, according to the Western calendar. In Chapter Twenty-two of Kṛṣṇa, the Supreme Personality of Godhead, Śrīla Prabhupāda comments that the gopīs “first ate haviṣyānna, a kind of food prepared by boiling together mung dāl and rice without any spices or turmeric. According to Vedic injunction, this kind of food is recommended to purify the body before one enacts a ritualistic ceremony.”