In the town of Siena:

Summer: the most active season. Its 27° and 32° and even 38º Celsius do not disappoint travellers choosing Tuscany. The season is great for biking, except during the peak time of the highest temperatures.

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. The ones who venture to come in this season can enjoy some festivals held in December.
Siena’s time zone is CET, UTC+1.

The main tourist period stretches from mid-April to the end of September.

For those wishing to explore Tuscany beyond the boundaries of Siena an overview of the rest of the region would include this information.
Weather differs around the region as it is not a uniform one as to landscapes. The coastal strip as well as the valleys are hotter than the hillscape which experiences rainier days. There are slight and wide variations from area to area. Yet if an adjective to describe Tuscany has to be chosen… mild is it.
April to June (spring) and September to October (autumn) are the most recommendable periods as the weather is better, open air activities abound and streets are not full to overflowing. Also prices do not soar as in high summer season. There is a full twist when in high season prices become somewhat unreasonable and it is very hot.
One fact is certain July and August are hot and short of rain, even they can even be scorching hot, obviously worse in big towns. Besides heat these months become a hive of activity when the Italians and foreigners make Tuscany their holiday destination. July’s temperatures range from 23 to 30 ºC. 38º and 40 ºC are not rare in the valleys.
Changes arrive in the middle of August when rain lowers the temperature and makes days fresher. There are still hot days in September and October ( autumn) nears the best moment to visit Tuscany, as the weather is clear, dryer and sunnier and cities are not overflown.
In November, autumn days experience lower temperatures and the olive harvest takes place in Tuscany, an activity that some travellers take part in.
Tuscan winters are cold and wet especially at night in the hills. January’s temperatures (the lowest throughout the year) range from 3.5°C to 5.5°C inland and reach 7ºC in the coastal towns.

To find out the weather in Siena link http://uk.weather.yahoo.com/italy/tuscany/siena-724196/