Allāh the Most High says (translation of the meaning): You certainly know those amongst you who transgressed in the matter of the Sabbath (i.e. Saturday). We said to them: “Be monkeys, despised and rejected.” The tafsīr (exegesis) of this noble Āyah (verse) is: You (Jews) know - what happened to your ancestors from the town whose inhabitants disobeyed Allāh when Allāh had ordained for them to honour the Sabbath. They did a tricked to fish on Saturday, by setting nets and digging pools, then they caught them on Sunday. This was a trick that they used to do something prohibited. Because they did that, Allāh transformed them into outcast monkeys.
[Al-Tafsīr Al-Muyassar (Simple Exegesis)]