On the 342 Mondays since the beginning of 1985 when the TSE 300 has lost ground, it has lost an average of 0.61 per cent. That's more than the down-day averages for the other weekdays. Moreover, when Monday decides to have a very bad day, it is typically a doozie. Of the 55 days when the TSE 300 has lost more than two per cent since the beginning of 1985, fourteen of them have been Mondays. Allowing for the fact that the market is closed more often on Mondays than on other weekdays, such big down days were almost 50 per cent more likely on Mondays than during the rest of the week.
Markets typically respond more dramatically to bad news than they do to good news.
Given that the weekend gives more opportunity for more news, it is more likely that the markets will open on Monday with some bad news to push the markets down. I'm open to other theories about why Mondays are stock-market underperformers, so send 'em in. And they do. Tuesdays after a holiday Monday produced an annualized loss on the TSE 300 of 9.9 per cent from 1985 to now. Tuesdays saw the TSE 300 shed more than two per cent of its value.