What prompts me to say this is that I follow a good handful of Krishna-related Facebook Pages, and according to one of them, today is evidently something called "Ekadasi", a holy day in which Hindus fast from certain foods. According to Stephen-Knapp.com, Ekadasi is as follows:
"Ekadasi generally falls on the 11th day after the new moon, and the 11th day after the full moon. Eka means one and dasi is the feminine form of dasa, which means ten. Together it means eleven. Only occasionally may it fall on a different day. So it is on these days that devotees and devout Hindus will follow the vow of Ekadasi and not eat any beans or grains, or products with such substances in them. Thus, the diet is expected to be simple and plain as part of the mood of renunciation, and preferably only once in the day if possible. Other recommendations include that the food should be made of vegetables, fruit, water, milk products, nuts, sugar, and roots that are grown underground (except beet roots). Restrictions include spinach, eggplant, asafetida, and sea salt, but rock salt is alright."
Elsewhere, it states:
"If one doesn’t follow Ekadasi because of illusion, he is still considered the utmost sinner. For every mouthful of grain that is eaten by a resident of the earthly region (on Ekadasi), one receives the effect of killing millions of brahmanas. It is definitely necessary that one give up eating grains on Ekadasi. I very strongly say again and again, ‘On Ekadasi, do not eat grains, do not eat grains, do not eat grains!’"
I think it's a personal decision as to whether one wants to fast, however to state that a person is outright sinning if they don't observe holy days and/or fast from certain foods at certain times is negating what Jesus took care of through His sacrifice on the cross.
Colossians 2:16-19 says,
Romans 14:1-4 states,
"One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind. He who regards one day as special, does so to the Lord. He who eats meat, eats to the Lord, for he gives thanks to God; and he who abstains, does so to the Lord and gives thanks to God. For none of us lives to himself alone and none of us dies to himself alone. If we live, we live to the Lord; and if we die, we die to the Lord. So, whether we live or die, we belong to the Lord. "