بسم الله الرحمن الرحيم
(Fatwa: 542/478/SN=5/1438)
It is lawful without any doubt to read Quran or tasbihat or giving charity etc and do isal-e-sawab for some deceased person after his death, but it is not proved from the Holy Quran and Hadith to gather on some fixed day or date (for example on the third day) and read Kalimah Shahadah or any other wazifah with the intention of isal-e-sawab. Rather according to Ulama such type of gathering is bida’h; hence you should not join such types of events as it is not lawful.
ويكره اتخاذ الطعام في اليوم الأول والثالث … واتخاذ الدعوت لقرأءة القرآن وجمع الصلحاء والقراء للختم أو لقراءة سورة الأنعام أو الإخلاص (رد المحتار على الدر المختار 3/148، ط: زكريا)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband