Tanzania is located in the East of Africa, bordering Kenya and Uganda. Due to its location on the equator the climate goes through dry seasons and wet seasons. Generally you can split the seasons into two parts.
December to March is the summer and March till May is the winter season. However, the ideal time to go is dependent on what you want to do and see so have a quick read before you look into hotels in Tanzania. You can also visit at http://aaafrica.net/ to get more data related to Tanzania safari tours.
December to February is the hot, dry season. February marks the start of the calving season for the wildebeests, during this time temperatures reach a moderate 20 degrees to a low 30 degrees. Therefore it is recommended that you wear mainly cotton cloths as well as sun glasses and hats for health reasons.
Coming to the end of February through to March there are a lot more variations to the climate. As the weather begins to cool (low 20's) there will be intermittent showers, increasing into April and May. March is however one of the better times for Game viewing.
April and May is the seasons of heavy rains. Roads become difficult for travel due to the heavy rain as such travelling is not really recommended unless you go with a seasoned tour group. Additionally the rain falls tend to start early morning and clear up by the midafternoon, so don't worry about being stuck inside too much!