Launcelot’s mention of Ash Wednesday and Black Monday indicates his little knowledge in the religious spheres. In his speech he said that both the occasions fell on the same time, which in reality is not true at all.

In fact, there is a period of 40 days between them. He simply mentions them to present himself as a clever witted person in front of his “cruel” master. Moreover he does so to play with Shylock’s superstitious nature, to simply tease him.