The Day should be understood as threefold. The Night has been ordained to be threefold. So also are fortnights, months, years, seasons, and conjunctions.
अहस्त्रिधा तु विज्ञेयं त्रिधा रात्रिविधीयते। मासार्धमासवर्षाणि ऋतवः संधयस्तथा॥