Mark 2:18
And the disciples of John and of the Pharisees used to fast: and they come and say unto him, Why do the disciples of John and of the Pharisees fast, but thy disciples fast not? And the disciples of John and of the Pharisees used to fast: and they come and say unto him, Why do the disciples of John and of the Pharisees fast, but thy disciples fast not?
The question was asked at a time when the disciples of John and of the Pharisees were fasting, while the disciples of Jesus were not. It came as an inquiry about why this difference existed between the two groups. John's disciples at that moment were in grief because John was in prison.
They were also uncertain and unsure regarding the work of Christ. Like the Pharisees, they were following rabbinical rules and seeking to be justified by works of the law. In that system fasting was practiced as an act of merit to earn favor.
Among the Jews the strictest observers fasted two days in each week.