I am in Mumbai, there was a discussion on the chaturmasya vrata mainly the Sannyasins do, during the rainy season. It is normally done for 2 months though the word chatur means four. They don’t cross a village / stream and stay put in one place teaching or discussing the scriptures.
How come a four months penance is reduced to two months. There is a Adhikarana (nyaya / topic) in poorvamimamsa where for a yaga which is to be performed for 120 years or so. Since it is impossible for a person to live for that long. And even if we argue for the sake, still one should spend atleast the first 20 years to gain the eligibility to perform the yaga, like being initiated in the sacred thread at an appropriate time, studying the Vedas, getting married and getting a male progeny. So after this if one starts the yaga, he will not be able to finish it. And if the sankalpa (determination) is done for a particular yaga it should be performed well, for the complete result. And, if not done properly or is given-up half way through, may yield wrong results. So, the final verdict will be, one cannot perform it in his life time. And, it cannot be passed on to the son etc., nor can one accept the statement by the veda to be redundant. If it is accepted redundant / useless in one place, the other portions also loose its pramanatva (the means of right knowledge-ness).
So, what is the way. Here the discussion spans in accepting the year to be a month, paksha (fortnight), week, day, hour etc. it can be performed in 120 months, therefore the other states, like, the fortnight, week etc, can be dismissed. And accepting this, the famous stamen, “samvatasaro vai masa:” – Year is a month, is made.
What is the relation of chaturmasya and this ? accepting this logic, it is accepted, we can deduce “maso vai paksha” – Month is a fortnight. Therefore, 4 months means 4 fortnights. Now, what is the pramana, and if it has no pramana, then why not reduce it to 4 weeks or days or even hours or minutes. Because the veda declares “chaturmasya yajinasya akshayam sukrtam bhavati” – the performer of chaturmasya will gain incomparable wellbeing / goodness. Now, anyone will say, just wait for five minutes, let me perform the chaturmasya yaga J. And one can perform more chaturamasya and gain greater greatness “adikasya adikam phalam” – greater the effort greater the result.
So, without a pramana nothing is accepted and here too what is the pramana, for accepting it to be 4 fortnights. Some say narada to be a pramana, he it seems performed the chaturmasya in 4 fortnights. This is not tenable, for Narada is a devarishi we ordinary mortals cannot compare ourselves with him. And, most importantly, if narada performed it for 2 months (even if accepted), his 1 day is ~ 1000 – 4000 days (I am very weak in these calculations, anyway, why should we be bothered about their time scale) , and it is not a general rainy season in all the place in the universe J. So, cutting short using this logic is not reasonable.
All said, according to the smritikaras (people who wrote the smritis), if there is a tradition then it should be accepted.