The very idea of releasing movies stems back to the 19th century plays. As plays became more and more accepted and entertaining, the idea to get more people to watch it and eventually commercialize it grew by the day. Plays and eventually movies were released on holidays or on festive days, when people wouldn't be too busy with their occupation.
'Ben-Hur' widely regarded as the first big blockbuster of the 'film' movie era, was shown continuously on holidays/Sundays and eventually saw spikes on these days. As global industrialization grew and work orders got formal, Sundays and more recent times Saturdays became huge earning days for movies and for any fun activities in General.
Fridays eventually became the 'run-up' day towards weekends, which were the high grossing days. Well producers do release on Thursdays too to cash on additional grossing days. (Eg: Bollywood's latest blockbuster PRDP was released on Thursday, leading to the ultra record collection owing to Diwali. The latest Bond movie also released on Thursday)