Learn Portuguese (Portugal): Eu vou às compras ao sábado.

In Portuguese, the standard expression for "to go shopping" is ir às compras (literally "to go to the purchases"). Because compras is a feminine plural word, the preposition a (to) merges with the feminine plural article as (the) to create the contraction às.

Why is it ao sábado instead of using a word for "on"?

To say you do something on a specific day of the week, Portuguese uses the preposition a combined with the definite article (the). Because sábado is a masculine word, a combines with o to make the contraction ao.

The English translation says "on Saturdays" (plural), but sábado is singular here. Why?

Using ao with the singular day of the week (sábado) is a very common way to express a recurring habit, meaning "every Saturday" or "on Saturdays". You could also say aos sábados (plural) and it would mean the exact same thing.

What verb does vou come from?

Vou is the "I" form (eu) of the highly irregular verb ir (to go). It is used here to mean "I go", but it can also mean "I am going".