The fasts, proposed and observed by parliament in the first half of the 17th century, have always been defined as opportunities for propaganda. This article focuses, instead, on their cultural and religious meanings: why MPs believed that the act of fasting itself was important and what they hoped it would achieve. It argues that fasts were proposed for two reasons: to forge unity between parliament and the king at a time of growing division, with the aim of making parliamentary sessions more productive and successful, and to provide more direct resolution to the nation's problems by invoking divine intervention. Fast motions commanded widespread support across parliament because they were rooted in the dominant theory of causation – divine providence – and reflected the gradual conventionalisation of fasting in the late 16th and early 17th centuries. However, this consensus seemed to wane in the early 1640s as divisions between Charles I and some of his most vocal MPs widened, while the fast day observed on 17 November 1640 was used by some MPs to express their opposition to Charles's religious policy, especially regarding the siting of the communion table/altar and the position from where the service was to be read. The article concludes by reflecting on how a study of parliamentary fasting can contribute to wider debates on commensality and abstinence.