Summer: the most active season. The temperature is typically hot, but is good for being outside. It is also great weather for biking.
Autumn: excellent weather conditions for travelling as average temperatures range from 24º to 21ºC. Favourable temperatures allow different festivals to take place.
Spring: also a recommendable season. It can become wetter than the rest of the year. Average temperatures range from 18º to 24ºC. This season gives visitors the chance to ride hot balloons and enjoy festivals held on the streets of Siena.
Winter: Temperatures fall as low as 10ºC. Streets become less crowded with visitors. Those who come at this time can see some great festivals in December.
Siena’s time zone is CET, UTC+1. The main tourist period stretches from mid-April to the end of September.
To find out the weather in Siena link http://uk.weather.yahoo.com/italy/tuscany/siena-724196/