Hayase has gone to the same school for years, so she knows that you do *not* touch anyone on Monday, and if you do, there's a little incantation that must be recited. But Mashiro has just transferred in, and does not know these things. Hayase explains that we do not touch on Monday, because not too long ago two girls did - and they fell in love, became lovers and tried to die together. Therefore, if you don't recite the charm, you might fall in love with whomever touched you...and that would be bad. Of course Hayase explains this after Mashiro has touched her...on a Monday.

(Source: Okazu)