Shashthi (Sanskrit: षष्ठी, romanized: Ṣaṣṭhī) also referred to as Chhath is the sixth day of the lunar fortnight in the Hindu calendar. It is tithi (lunar day) of a paksha, the fourteen-day phase of the moon.

The word comes from the Sanskrit cardinal ṣaṣ (six), whence the ordinal number (linguistics) ṣaṣṭha (sixth), fem. ṣaṣṭhī (days of the paksha are feminine gender).

The sixth tithi, especially in the waxing period (shuklapaksha), is important in several rituals including: