Xian can be visited all year round, but the best times to visit are spring and fall. In summer it can be very hot and humid. Winters are short, but cold.
Xian is a safe city. You do not need to take any special precautions with your belongings. From time to time you may be approached by locals who will tell you they want to practice their English. Do not be alarmed. Although some of them may have a souvenir shop and will try to take you there, most of them have a real interest in the English language.
No matter which foreign country you come from, if you do not look Chinese, locals will think your English is fluent. This is good news, because it makes meeting local people very straightforward. I even met a policeman in a bus who asked me some (simple) questions in English.