The different museums are:
Palacio Fernan Nuñez, Calle Santa Isabel 44, Metro Atocha - Anton Martin, free days: 9th, 16th, 23rd and 30th.
Real Academia de Bellas de Artes San Fernando, Calle Alcala 13, Metro Sol, free days: 27th only! Opened between 9:00 and 15:00 tuesday - saturday and sundays 9:00 - 14:30.
Museo Lazaro Galdiano, Calle Serrano 122, Metro Ruben Dario, Gregorio Marañon, Avenida de America, free days: 10th, 17th, 24th and 31th. Otherwise its free between 15:30 - 16:30 and on sundays 14:00 - 15:00 , its opened between 10:00 - 16:30 monday, wednesday - sundays.
Museo del Romanticismo, Calle San Mateo 13, Metro Alonso Martinez, free days: 7th, 8th, 11th and 18th. On Its closed on mondays and on saturdays its free entrance after 14:00 if you can't make it 7th, 8th, 11th and 18th. Other days entrance 3 euros.
Museo Cerralbo, Ventura Rodriguez 17, Metro Plaza España, free days: 13th, 14th nd 15th. Closed mondays and opened between 9:30 and 15:00 Thursdays 17:00 - 20:00. Free entrance saturdays after 14:00, thurdays and sundays all day. Otherwise 3 euros, 1,5 student card, +65, -18.
Museo Sorolla, Calle General Martinez Campos 37, Metro Gregorio Marañon, free days: 25th, 26th, 28th and 29th. Tuesday - Saturdays 9:30 - 20:00, sundays 10:00 - 15:00 and free entrance. Students, -18, +65 etc. free entrance always.
Museo Nacional Artes Decorativas, Calle Montalban 12, Metro Banco de España - Retiro, free days: 20th, 21th and 22nd. 3 euros/1,5 euros. Tuesday - saturday 9:00 - 15:00, thursdays 17:00 - 20:00, sundays 10:00 - 15:00. Free entrance sundays.
More info here.