I stumbled onto, and subsequently forgot about, and today remembered about, a very handy resource for checking out historical weather conditions. Lets say you're going on holiday (in this instance, to Prague) in June this year, you can easily look up what the weather was like on any given day or week in June last year, which in theory will give you a very good idea what it's going to be like again this year. (global warming aside...)
The website is here: wunderground.com/global/stations/11518.html , and shows the Prague weather for today. If you scroll down and look to the left, at the 'History and Almanac' box, simply put in the time of year you want to see for last year (or any of the last 15 years or so), and click on view; and then you can view the daily, weekly, or monthly views, which as I say in theory should give you a good indication of what the weather should be like for your holiday in this current year.
Hopefully people find this as helpful as I did.