Unless there's some unforeseen bombshell affecting international finance (like the US government announcing that the USA is officially bankrupt), you can be reasonably confident that the exchange rates used by the international banks won't change much between now and Saturday.
It's far more important to consider where you'll change your money than when. Generally, you'll get the best rate of exchange by using your debit or credit card at an ATM in France. Some banks charge for overseas withdrawals but, even allowing for those charges, you'll almost certainly get a better deal than changing your money in the UK, or using bureaux de change in France. (Some people open a separate bank account, which doesn't charge for overseas withdrawals, just for their holiday money. The Nationwide FlexAccount is probably the most popular although, of course, it would be too late now to get a new ATM card for your forthcoming trip).
If you want to take some Euros with you, many people recommend Marks & Spencers for the best rates. If you're not close to a M&S bureau de change, the Post Office is frequently recommended for giving a fairly good exchange rate.
Whatever you do, avoid the exchange desks at airports, whether in the UK or in France.
Chris