If we knew for sure that euros are only going to be more expensive in the future, then the answer would be easy: Buy them all at one time, so that we are getting them at the best price. Of course, we can't assume that to be the case, they could get cheaper, so the answer gets more complicated.
Focusing strictly on monetary considerations, there are two factors to examine:
- List item The overhead cost of currency conversion. How much do we anticipate each conversion costing?
- List item The variation in exchange rate. Can we assume the rate do remain steady or is it more reasonable to assume fluctuations? If there are fluctations, how much might it change over the course of a year?
Using answers to this Travel Stack Exchange question as a reference, you see that the cost of currency conversion can be as low as 1%-2% if you make the transaction with a debit card, but can be as high as 15%. So, buying 1000 euros a month would cost between 20 and 150 euros.
Examining a two year chart of the Euro-Canadian Dollar exchange rate gives us an idea of how much the currency fluctuates. Over the past two years, a euro has cost has much as $1.54 CAD and as little as $1.26 CAD, a 22% spread. Looking at it on a month-to month basis, we see that monthly changes have been as high as .05 to .07 (4-5%). As such, buying 1000 euros a month could cost 50 CAD more (or less) on a monthly basis due to variance in the exchange rate.
If we anticipate our overhead cost of currency conversion to be more than 5%, it doesn't make sense to do multiple transactions; the costs are likely to outweigh the benefits. If we can keep them under that amount, then multiple transactions are advantageous when the euro is cheaper.
The problem is somewhat analagous to that of someone who wants to make an annual investment in a mutual fund and is unsure of whether to make the purchase all at once, or to divide it over multiple purchases. One can't know for sure which way the mutual fund price is going to move over the time period Dollar cost averaging, spreading the purchase over regular intervals, is the generally accepted solution to this problem.
As such, so long as we can keep the overhead cost of currency transactions low (<5%), doing transactions on a regular basis positions ourselves to take advantage of possible drops in the price of euros and reduces the risk of buying euros when they are most expensive. If we can't keep the cost low, then currency fees would be greater than potential price drops and we would be better off doing a single transaction.