Minas Gerais is generally hotter (high 30Cs) with heavier rain and higher humidity from November to March. Autumn/winter (May to September) is drier, but it gets much colder here than in neighbouring states, with temperatures sometimes dropping to 5C.
There are several direct flights each day to Belo Horizonte (the state capital) from Rio and São Paulo. From Belo Horizonte it's a 1.5-hour drive to Ouro Preto and a 3-hour drive to Tiradentes. You can also fly from Belo Horizonte or Rio de Janeiro direct to São João del Rei with TRIP Linhas Aereas; from there it's a 20-minute taxi ride to Tiradentes.
If you want to travel overland, it's a 5-hour drive from Rio to Tiradentes and a 7-hour drive from Rio to Ouro Preto. Several buses leave daily from Rio to São João del Rei, with onward connections to Tiradentes.