Shadow of a 1 m vertical stick at local noon. Plan your activity for the highlighted day, but the days either side are nearly as good.
The Sun's declination — how far north or south of the celestial equator it sits — swings between about +23.44° in June and −23.44° in December. On the day the noon declination equals your latitude, the Sun at local noon is at the zenith and a vertical pole casts no shadow.
Every place between the Tropic of Cancer and the Tropic of Capricorn gets two such days a year, one as the Sun heads towards the tropic and one as it comes back. Exactly on a tropic the two merge into one, at the solstice. Outside the tropics the noon Sun never reaches the zenith.