The weather in Paris is good for visiting any time of year. The summers are warm with average daytime temperatures of 25°C with only the odd day reaching stifling heat of 32°C or more. The winters are fairly mild compared with the UK with an average daily high of about 7°C and temperatures only dropping below freezing on the odd night. Rainfall in Paris is quite high but mostly comes in the form of isolated showers so be prepared with a brolly and nip into an obliging café to pass the time.
As one of the most popular tourist destinations in the world, Paris doesn't have a true low season but winter visitors will find the city a lot quieter and enjoy less queuing for the top attractions. April to July are the peak of the tourist madness. At the end of July and throughout August the majority of the locals go on holiday, resulting in a quieter Paris. Some shops close for this period, although most tourist attractions will stay open. Christmas and Easter are also busy times in Paris but if you choose to travel in early spring or late summer you can enjoy the best of the weather and fewer tourists.
It might be a short hop away from the UK, but Paris is a destination worth planning well in advance to get the best deals. You need to book Eurostar a couple of months in advance in order to get the cheapest tickets, as prices increase steeply for last-minute trains. The same is generally true of flights. Hotel prices stay the same most of the year round, though you are more likely to find a deal during the quieter winter months.