Create a view and populate 1 column with the extracted day-month from a epoch date from the main table
The answer is, like you already mentioned in the PostgreSQL documentation. An example is in the reply of Postgres: how to convert from unix epoch to date?

select timestamp with time zone 'epoch' + Day_start * interval '1 second'
from   "MainTable";


or simpler (thanks to @a_horse_with_no_name):

select to_timestamp(day_start)
from   "MainTable";


Use the to_char(timestamp, 'DD-MM') to get the day and month. The result to put in your CREATE VIEW would be something like:

select to_char(timestamp with time zone 'epoch' + Day_start * interval '1 second',
               'DD-MM') AS index;
from   "MainTable";


If you want the whole date as DATE type then use:

select (timestamp with time zone 'epoch' + Day_start * interval '1 second')::date AS index;
from   "MainTable";