As a practice/notification one should go to temple and make alankaram for shubha sweekaram, This is because from that day the 13 days mailam will end. Also, the departed soul (Preta Atma) will be converted to Pinda Deha Sharir, which is of thumb nail of size. The pinda deha sharir needs to start his/her journey to Swarga or Naraka through Yama lokam. Yama will give entrance to narakam and Dharma will give entrance to Swargam. Now when the Pinda deha sharira has to start his journey he should given an intimation that we will give you the light and all other requirements while in the journey. Hence, these celebrations are observed. This is as per the Garuda Purana and Smrithi. The shodasa danam, nitya masikams, etc are the daily requirements of pitrus for the period of 1 year.
It is also explained in the purana that, pithru's one day is equal to 30 days of Bhulokam, i.e., when we give pinda in masikam, that will reach them on daily basis. for a period of 12 days (one year of bhulokam) they will reach yama lokam.
So, please be assured that these are not only told by our elders but it is told by Lord Vishnu to Garuda in Garuda Puranam in with conversation with Lord Shiva.
Hope this will suffice your need.
Arun