Question: Is fasting on Ekasdashi one kind of Karma Kanda activity?
Answer: The reason why devotees fast on Ekadasi is because Lord Caitanya wanted it.
Lord Caitanya is pleased when devotees follow Ekadasi fast.
In Caitanya-caritamrita, there is a section dealing with this topic.
Adi Adi-lila 15.8-11. Here Lord Caitanya instructs His mother Sacidevi to follow this fast.
If we do not follow this fast, we will get sins.
That does not make this a karma-kanda ritual.
By following karma-kanda rituals, one expects material rewards but devotees follow Ekadasi in order to obtain pure devotional service.
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