SUNDAY 15th Sept is actually Ekadasi but we don't fast on that day because the presence of dashami during the Brahma muhurta pollutes the Ekadasi, according to Sanatana Goswami. The GCAL agrees with this.
Monday 16th Sept is Dvadasi and so fasting for both the Parsva Parivartana Ekadasi and the most auspicious appearance day of Lord Vamanadeva is observed. The fasting for Lord Vamanadeva can be for just a half day and the feast must be strictly Ekadasi prasada only. See Ref 1. below. Monday is also Srila Jiva Goswami's appearance day. However, regarding fasting on Monday, Sanatana Goswami and Lord Krishna state that Parsva Parivartana Ekadasi is a more important nirjala fast than Pandava Nirjala. See Ref 2. below. (Ignore them at your own peril.)
Tuesday 17th Sept is the most auspicious appearance day of Srila Bhaktivinoda Thakur and fasting is observed until after the midday puspanjali and kirtan in his honour.
Wednesday 18th Sept is Ananta Caturdasi Vrata and the disappearance day of Srila Haridasa Thakur. According to Srila Prabhupada we should fast till Sunset on Ananta Caturdasi, see Ref 3. below.
Tapasya is the wealth of the brahmanas.
Ref 1. No grains on Vamanadeva's appearance:
“You should describe the ritualistic duties to be performed every day, and you should also describe the fortnightly duties—especially how to observe the fortnightly Ekädaçé fast. You should also describe the duties to be observed every month, and you should especially describe the observance of ceremonies like Janmäñöamé, Räma-navamé and Nåsiàha-caturdaçé." (C.C. Madhya 24.341)
Note that Vamanadeva's appearance is equated with other days on which we cannot take grains.
Ref 2. The 3 must nirjala Ekadasis.
“If an initated Vaisnava, who is engaged in devotional service of the Lord, eats grains, or even fruits and roots, on Sayana Ekadasi, Parsva-parivartana Ekadasi, or Utthana Ekadasi, I do not forgive his previous or present offenses and I will place him into the darkest region of hell.” (HBV 12th Vilasa Text 101-102)
“If one eats fruit or roots or drinks water on Sayana, Utthana, or Parsva-parivartana Ekadasis, it is considered by Me to be like piercing My heart with an arrow.” (HBV 12th Vilasa Text 103)
Lord Sri Krishna says "He who swallows food on Utthana, Sayana, or Parivarthana Ekadasi sends Salya into my throat.” (Kalapraksika Ch XLV)
For the skeptics looking to discredit the validity of Sri Hari Bhakti Vilasa (and I come across them all the time) Sri Hari Bhakti Vilasa is one of many books Srila Prabhupada wanted to be translated for his ISKCON (see SP Siksamrita).
Note that the above 3 nirjala Ekadasis are divine celebations as follows:
1. Sayana Ekadasi - The Lord goes to sleep in the ocean of milk.
2. Parsva Parivartana - The Lord rolls over in His sleep in the ocean of milk.
3. Utthana Ekadasi - The Lord arises from His divine sleep.
It is stated that during the above four months of Caturmasya no yagnas can be performed because the Lord Vishnu and the devas are all sleeping and will not therefore accept the offerngs. But the most auspicious yajna of all, the sankirtana yajna, can be performed at any time. But weddings, for example, and other such necessary mundane ventures are not recommended during that time.
Ref 3. Fasting on Ananta Caturdasi
Prabhupäda: Yes. Bhaktivinoda Öhäkura 13th, and 14th Ananta Caturdasi and Haridäsa Öhäkura. So, this 14th September should be observed fasting, whole day. [those dates relate to 1972. Fasting whole day refers to Ananta Caturdasi]
Devotee: Fourteenth is the advent of or the demise of Haridäsa Öhäkura?
Prabhupäda: Yes, but not for demise of Haridäsa Öhäkura, but because that is Ananta Caturdaçé Vrata, the, generally the fasting is observed till evening, up to 5 o'clock. The next day, Viçvarüpa Mahotsava, feasting. (11th March 1972 Vrindavana Room Conversation)
Conclusion is that on Ananta Caturdasi Srila Prabhupada wanted us to fast the whole day unit Sunset.
Austerity is the real wealth of the brahmanas and of course it makes chanting Hare Krishna - our most important activity - much easier.
Hoping this meets you well. Hare Krishna!
Ajamila Dasa acbsp