IMO the best time to buy travel money (unless you have a crystal ball) is when you arrive at your destination using mastercard or visa which you have put into credit with your own money so that it is not treated as a cash advance...the ATM spits out the local currency and actually gives you a better conversion rate than what you would get if you converted cash...and it doesnt matter if you take out $20 or $500 the rate is the same....try changing a $50 US note here, see how shockingly they rip you off.....much much more than a week or two or more of normal currency fluctuations.....also you get paid interest on the funds that you have in the card. Its also imo more secure then carrying a wad of cash around with you converted or not. Hope this helps.
