Not to keep fast only on Saturday.
(Abu Dawood, Tirmizi, Sanad Hassan)

However, it is much better to keep a fact on Sunday along with Saturday, so that there is a display of dissidence to the followers of the Book (Jews and Christians), for they consider both these days as Eid.
(Musnad Ahmad, Sanad: Hassan)

It is very good, if those living in Medina go to Al Masjid ul Quba is on every Saturday and offer their two rikaats of salat at the time of Ishraq.
(Saheeh Bukhari, Saheeh Muslim)