Pandava Nirjala Ekadashi

During the hottest season (northern hemisphere), if one can perform upavasa without water one is performing a great feat.
One may take only caranamrta and acamana water.
Bhima requested one vrata day by which he could obtain the fruits of all the other days of vrata that he was incapable of observing.
Vyasadeva advised him to follow this difficult vrata.
One should worship the Lord in the evening by bathing him in milk.
On the dvadasi one should give full pots of water to brahmanas and feed them before breaking one's fast.
SAMVATSARASYA YA MADHYE EKADASYO BHAVANTI HI TASAM PHALAM AVAPNOTI PUTRA ME NA ATRA SAMSAYAH ITI MAM KESAVAH PRAHA SANKHA CAKRA GADADHARAH
(HARI BHAKTI VILASA 15/25 from PADMA PURANA Vyasadeva speaks to Bhimasena)
Oh son, Lord Keshava, Who holds the club, disk, conch and lotus flower in His hand, personally told me that all of the merit achieved by fasting on whatever Ekadasis fall in one year can be attained by fasting on this one Ekadasi (nirjala Ekadasi).
Of this, there is no doubt.
ATMADROHAH KRTASTES TU YAIR ESA NA HY UPOSITA PAPATMANO DURACARA DUSTAS TE NA ATRA SAMSAYAH
(HARI BHAKTI VILASA 15/33 from PADMA PURANA Vyasadeva speaks to Bhimasena)
Anyone who does not fast on this particular Ekadasi (nirjala Ekadasi), they should be understood to be sinful, corrupted and suicidal person without a doubt.
The Story of Pandava Nirjala Ekadasi:
Once Bhimasena, the younger brother of Maharaja Yudhisthira, asked the great sage Shrila Vyasadeva, the grandfather of the Pandavas, if it is possible to return to the spiritual world without having observed all the rules and regulations of the Ekadasi fasts.
Bhimasena then spoke as follows, "Oh greatly intelligent and learned grandfather, my brother Yudhisthira, my dear mother Kunti, and my beloved wife Draupadi, as well as Arjuna, Nakula and Sahadeva, fast completely on each Ekadasi and strictly follow all the rules, guidelines and regulative injunctions of that sacred day.