The currency for Portugal is the Euro. In the ALGARVE  region  you can buy/exchange Euros very easily at Banks,  Cambios (Bureau de Change), Hotels and even shops.  The best rates and commissions are normally found at the bureaus in larger towns, the best thing is to keep an eye out whilst out shopping and always check the commission rates. You will normally need your passport to exchange cash or travellers cheques. 

There are automated cashpoints (Multibanco) now at most of the larger supermarkets and in the main shopping areas of most towns. Instructions are available in English if required.

You can also find Multibanco machines  in every small town and even villages all around Portugal. VISA, Mastercard and American Express cards are widely accepted in hotels, shops and restaurants. Petrol (gas) stations usually take credit cards and cash.

 Banks do not open at weekends. Opening hours vary and are posted clearly outside. but usually banks will be open between 0830 and 1530. In smaller towns a bank may close for lunch. Many branches no longer offer a foreign exchange service.

If you can, book all of your hotels on-line from home, and pay for them in advance in your local currency (via a site like expedia or hotels.com for example). That way you avoid fees from your credit card company or the those charged by obtaining cash from ATM machines.