The ideal time is during the Aparahna kala (afternoon phase, generally between 1:00 PM and 3:30 PM), or as prescribed by your regional almanac (Panchangam). It should never be performed during early dawn, dusk, or night.
Disclaimer: This article is for educational purposes. Always consult a qualified priest (Brahmin Vadhyar) for your specific gotra and shakha before performing any Vedic ritual. yajurveda amavasya tharpanam in sanskrit pdf full
Tharpanam must be completed during the daytime hours, preferably during the Kutapa Kaala (late morning to noon window). The ideal time is during the Aparahna kala