Sometimes, due to the manner in which the ekadasi tithi and the dvadasi tithis are positioned in relation to the sunrise (there are eight different cases), the vrata is observed on the dvadasi and the fast is broken the next day, the trayodasi.
Such a fast day is called a Mahadvadasi, to distinguish it from a regular dvadasi, which is the day to break the ekadasi fast.
Regarding your questions about the calendar-almanac:
1. Herapancami is correct (9 July 1970).
2. When there is an important Dvadasi, the Ekadasi fasting is transferred on the Dvadasi, and this is called Mahadvadasi. The 26th October, 1970, is actually Dvadasi, so the fasting is observed together, or Ekadasi fasting is disregarded, and the Dvadasi (Mahadvadasi) fasting is taken as important.
Srila Prabhupada:Letter to Pradyumna — Los Angeles 29 April, 1970:
If Mahadvadasi is in force, fasting is prohibited on the Ekadasi, but the observance is done on the Dvadasi.
As already said there may be several reason, amongst which there are:
1. If Ekadasi begins after sunrise and ends before sunrise the next day it is considered lost, too short, thus not full, and impure. Therefore the next day is called Unmillani Mahadvadasi.
2. If Dvadasi begins after sunrise and ends before sunrise on the next day (Trayodasi), it is also Lost, and is to be observed as Trisprsa-Mahadvadasi.
3. If Dvadasi falls on the Sunrise two days in a row the first Dvadasi becomes Vyanjuli Mahadvadasi.
4. When the following Amavasya or Purnima falls on sunrise two days in a row the preceeding Dvadasi becomes Paksa-Vardhini-Mahadvadasi, Paksa based.
– Manonatha Dasa (ACBSP)
29 May 2019