Prices fluctuate constantly, more than even daily although most pricing is daily.
The Wall Street Journal has 30/60/90 day forward contract prices. Experts bet on which direction currency trading goes and I have strongly warned posters to previous similar topics that betting on currency fluctuations is best left to them.
Here are some suggestions:
If you owe money on your credit card, rather than exchanging money now or pre-buying, pay off some of that debt. It is better to pay of something costing you 20-25% annually than betting the dollar will go down by 10%.
If you really think the dollar is dropping more, use your money to pre-pay something where it is refundable or dates are flexible. Otherwise, you will lose far more pre-paying if you have to cancel than any dollar fluctuation.
Consider next pre-paying something that is sure to rise no matter what especially if that item was priced low before this year's euro rise - like railpasses.