Quite a lot of people from colder countries, eg in northern Europe, like to visit Portugal in the winter and early spring because it is warmer. However, this time of year in Portugal can also be cold and wet, so if you are looking for a sunshine holiday you should think carefully. In that case it would be better to think about the Madeira area, which is mild and warm more or less all the year round, or the Azores rather than the northern regions (though even these are warmer than northern Europe). You can get a general idea of temperatures here.
Many people like to visit in the spring or early autumn, when the weather is usually pleasant in most of the country and there aren't so many visitors. While summers can be very hot, especially in the north, and many tourists like this, the popular resorts can also be very crowded in the summer holidays.
For further information on Portugal, including the activities you can do at different times of year, please click here.