Answered by Mufti Yusuf Mullan
Calling to Allah – why do some groups emphasize specific times to go out and specific methods? Are these from the sunna?
The reason why scholars do not attempt to “prove” these durations; four months, forty days etc., is simply because the time periods although important are not relevant at the devotional worship level. An addition to the forty days does not render it invalid. The significance attached is for organizational purposes only. Hence no need to establish it through specific references. The one of the greatest benefits one gains through going out to call others to Allah—through any Sunni groups—is in the deeds and righteous actions which are performed consistently throughout this period, and not in the specificness of the time-periods involved.
Given the wide-spread good seen in such work, one may suggest it to others as a means to self rectification. Self rectification will then, in turn, cause states to be favorable, as is obvious.
The statement regarding the interest bearing loan is rejected, and does not detract from the value of this effort.
Wa alaikum salaam,