"algunas" is formed with "algun+a+s"
where the "a" is the feminine and the "s" is plural.
Algun means "some": algun pais: some country, algun amigo: some friend
so algunas means some for femininine plural: some parties.
party, Fiesta is feminine in spanish, so you need a feminine version of algun.
and the sentence means: some parties are celebrated in the house (casa not case!) of the family
well, in this context, fiesta can also be a "celebration" or "holiday", not only a party
so it means "some unspecified ones"
La postura es recta, los gestos son amabilidad y sonrisa, los movimientos con las manos saludando con respeto.
