Not a great idea in theory, and completely useless in practice.

In reality, very few people actually withdraw their coins from exchanges on this day, and so any exchange operating a fractional reserve system can still easily cover the withdrawals. Even if everyone did withdraw their coins, publicizing a day in advance that you are going to withdraw your coins gives the exchange plenty of time to buy or borrow more bitcoin temporarily to cover those withdrawals, and then go right back to their fractional reserve practices the very next day. And even although some exchanges suspend withdrawals or go in to temporary "maintenance" mode during this time, people completely ignore the fact that doing so is pretty much proof they are running a fractional reserve system, and continue to use those exchanges anyway.

Rather than promoting a single day when people should withdraw their coins from exchanges, we should instead promote the idea that you should never have coins on exchanges unless they are actively being traded. Deposit, trade, withdraw, over as short a time period as possible.