The next day May 18th marks the International Museum Day. This day was established by the International Council of Museums (ICOM, UNESCO organism).
In Portugal, this day is also celebrated with many different activities. For example, on May 18th all Museums and Monuments dependent of the General Directorate of Cultural Heritage will open doors for free. 16 entries are free from 6 p.m.(See more information here;this link isn't written in english). Across the country will run similar initiatives.
This year the theme for this day is "Museums for a Sustainable Society - aims to promote awareness for the effects of human activity on the environment, and highlight the role of museums in the development of new methods of thinking and acting, guaranteeing respect the limits and the diversity of nature. ".
The Portuguese Heritage Affordable leaves you here one Museums list and monuments accessible to people with disabilities. Please, confirm the entrance conditions of the institution that you wish to visit.