Bhagwat Puran states that Solar and Lunar eclipses occurred within 13 days before start of Mahabharat. It implies that the fortnight or Paksh just before Mahabharat was of 13 days and that indicated a great ruin in near future. Mahabharat saw massacre of such a magnitude that people who survived could be counted on fingers.
In October 2005 there were two eclipses which were followed by the Kartik Shukla Paksha of 13 days from Nov. 3 to Nov. 15. In that fortnight pratipada and poornima, both were kshaya tithis. Astrology says that a fortnight of 13 days gives hardships to king and public both.
Let us see how this phenomenon occurs astronomically. The average period between 2 amavasyas is 29 days 14 hours 44 minutes and 3 seconds. This could be upto 6 hours less or more. The average duration of a fortnight is 14 days 21 hours and 22 minutes, however this may vary upto one day. Therefore a fortnight could be of minimum 13 days 23 hours to maximum 15 days 22 hrs.
Therefore when a fortnight is of lesser duration than 14 days, as tithi is counted from sunrise, the possibility of a fortnight of 13 days is created. If the amavasya is ending after few minutes of sunrise and Poornima is ending just before sunrise the fortnight is of 13 days. Similarly if the Poornima ends just after sunrise and next Amavasya ends just before sunrise the fortnight is of 13 days.
For example in Kartik Shukla Paksha, amavasya in Delhi ended on 2 November 2005 at 6.56 in morning while sunrise at 6.35 a.m. Therefore, 2 November was considered amavasya. Pratipada on 3 November ended at 6.26 a.m., i.e. before sunrise, so it was a kshaya tithi. Therefore the fortnight started on 3 November with Dwiteeya.
On the ending day of fortnight i.e. on 15 November, the chaturdashi ended at 7.28 in morning. Poornima too ended before sunrise at 6.29 on 16 November. This way, Poornima too, became a kshaya tithi. The difference of period between 16 November 6.29 hrs to 2 November 6.56 hrs gives the duration of fortnight as 13 days 23 hours 33 minutes which is only 27 minutes less than 14 days. However as the amavasya ended immediately after sunrise and poornima ended just before sun rise the fortnight was of 13 days.
The point to be noted is that duration of fortnight is only 27 minutes less than 14 days while the longitude of India is spread from 68° East to 98° East i.e. there is a difference of 2 hours in sunrise. Therefore the 13 days fortnight did not take place in whole of India. It happened only between 70° East to 80° East. If we take the example of Kolkata, sunrise occured earlier than ending of poornima and thus kshaya of poornima did not take place. Therefore, there the fortnight was of 14 days. Similarly in West, if we take the example of Dwarika, the sunrise occurs late, therefore amavasya ended before sunrise. If we assume starting of fortnight there from 2 November, the fortnight was of 14 days ending on 15 November.
This way, the 13 days fortnight is not applicable for the whole globe, rather it is significant to a particular area only. In India, this area covered Delhi and Mumbai and about 50% of India. Additionally, the duration of a fortnight is minimum 13 days 23 hours, therefore area experiencing 13 days fortnight is not more than 1/24 of earth.
One more fact worth mentioning is that occasions when the fortnight has less than 14 days average 3-4 times in one year. If we look at 2005, this happened thrice. From 25 March 26.28 hrs to 8 April 26.02 hrs the Chaitra Krishna Paksha was of 13 days 23 hours 34 minutes. Again Vaishakh Krishna Paksh started from 24 April 15.36 hrs and ended on 8 May 14.15 hrs, the duration being 13 days 22 hours 39 minutes. Third event was Kartik Shukla Paksha in November with a duration of 13 days 23 hours 33 minutes.
Whenever the actual period of fortnight is of less than 14 days duration, Paksh becomes of 13 days somewhere in the world. To know the location we have to ensure that the amavasya or poornima end at the time of sunrise. If end of amavasya/poornima in India is after sunrise then in West of India the fortnight will be of 13 days. If it is earlier than sunrise then in East of India, there will be fortnight of 13 days. The reason for this is that Sun rises late in West.
The above calculations also reveal the same phenomenon. The fortnight of 13 days occured in March 2005 in Japan, in April 2005 in New York and in November 2005 in India. The first two incidents were not reported in newspapers as they happened in foreign countries where tithis etc. are not practised.
Therefore it can be stated that a fortnight of 13 days occurs every year somewhere in the world. On an average this situation comes at one place after 20-30 years. Astrologically, it is a bad omen for the area where the fortnight is of 13 days, and indicates mass killings and lack of control in administration.