‘And it came about, when he went into the house of one of the rulers of the Pharisees on a sabbath to eat bread, that they were watching him.'
The description here is unusual for there were no rulers of the Pharisees. It may, however, merely signify that the man was both a ruler of the Jews, and also a Pharisee. Or it may suggest some privileged position among the Pharisees. The former is most probable. But Luke's aim in speaking of the Pharisee as ‘a ruler of the Pharisees' may be in order to suggest that that we are to see this house as like ‘the ruler's house' (Luke 12:36). And he is possibly to be seen as comparing with the householder of the parable in the chiasmus parallel (Luke 12:39) whose servants were expected to fulfil their duties (Luke 12:35-40).
(It is true that the parallel is not wholly exact, but the implications are all there. Exactness was not possible when the master of the house in the parallel was either God or the Lord).
As the servants were in the lord's house in the parallel parable, so Jesus has come into this man's house and is surrounded by those who would claim to be His fellow-servants. And here He eats bread with them. But the fellow-servants who surrounded Him were Scribes and Pharisees who were all watching Him. In this last regard it is possible that the sick man had been put there deliberately, but not necessarily so. The situation may simply have been that Jesus was under general surveillance, just as the servants were in the parable. Indeed the Scribes and Pharisees were under surveillance too, although they may not have considered the fact. But certainly as the Servant of the Lord Jesus knew that He was always under God's surveillance in order to see that He would do what was right.
The meal would be the main meal of the day following the synagogue service, a meal to which it was quite normal to invite guests. On the Sabbath there would be three meals, all of course cooked on the previous day (there were even instructions in the traditions of the elders on how and how not to keep it warm lest any ‘cooking' occur on the Sabbath), but the midday meal was the main one. On other days there would only be two meals and the main meal would be towards evening. Being in the house of a leading Pharisee there would be jars of water set apart there which provided ‘clean' water for the washing rites which all would be expected to observe.