If you want to come to the country and see a particular bank holidays, any time of the year will do. There are many holidays in Portugal and they are fairly evenly spread throughout the year. Whichever region you choose, you will find one in every region:
Wine Festival;
Sardine Day;
Carnivals (take place in February and last several days);
Chestnut festivals, etc.
The locals are very fond of this kind of entertainment and are happy to invite guests to attend.