The celebrations to which the Brit-milah takes place ends with the ceremony of the “third night” (tlatelila or el thelet lilah) – that is, the third evening after the circumcision. The “third night” of a milah that took place on a Saturday will therefore take place on Monday evening.
This custom, less followed today than in the past, was known in North Africa as well as in the Ashkenazi or Hasidic world [1]. It still takes place in some families, especially in the Hasidic and Tunisian communities.
This custom is mentioned in especially by two Tunisian chief rabbanim, R’ ‘Ouziel El’haik and R’ Israël Zeitoun [2].
Explanation
The third day after circumcision is considered the most difficult. After this day, the child is out of danger. The study evening of the third evening is therefore the counterpart of the night before the Brit-milah. This evening does not celebrate the end of the danger (it would be the fourth evening in this case) but allows those present to give the child an additional merit (therefore better protection) to get through this difficult day. This allows parents to increase their merits (by offering food to visitors during the reception meal).
Before returning Eliyahu hanavi’s chair
If the Brit-milah was carried out at home, then efforts will be made to return the highchair only after the third night.
Indeed, in honor of this third night, the chair is decorated with “rimonim” (metal objects, sometimes silver, usually decorating the Torah scrolls and featuring small metal bells – sometimes in the form of a grenade). This was the case in Tunisia.
It was said that in the middle of the third night after the Brit-milah, the bells of the “rimonim”were ringing at the exact moment when the prophet was leaving. In some families, the older children tried to stay awake to witness the departure of the prophet [3].
Informal reception meal (se’uda)
Even without having been formally invited, it is customary for friends and relatives to come to the baby’s home on the evening of the third night.
https://brit-milah.com/la-troisieme-nuit-tlat-leila-ou-el-thelet-lilah-coutume/
(c) R’ Eliyahu Bakish.