SB 5.21.4
yada mesa-tulayor vartate tadaho-ratrani samanani bhavanti yada vrsabhadisu pañcasu ca rasisu carati tadahany eva vardhante hrasati ca masi masy ekaika ghatika ratrisu.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the sun passes through Mesa [Aries] and Tula [Libra], the durations of day and night are equal. When it passes through the five signs headed by Vrsabha [Taurus], the duration of the days increases [until Cancer], and then it gradually decreases by half an hour each month, until day and night again become equal [in Libra].