Learn Portuguese (Portugal): Mudo os lençóis ao sábado.

Mudar and trocar can both mean to change. Trocar is often used for physically swapping one thing for another, but for changing bed sheets, clothes, or diapers, both mudar and trocar are very common and interchangeable in Portugal. You could also completely correctly say troco os lençóis here.

What is the singular form of lençóis and why does it change so much in the plural?

The singular is lençol. In Portuguese, nouns ending in -ol form their plural by dropping the -l, adding -is, and putting an acute accent on the o. That is how lençol turns into lençóis. Another example you might know is sol (sun) becoming sóis.

The English translation says on Saturdays (plural), so why is sábado singular in the Portuguese sentence?

To express a recurring habit on a specific day of the week, European Portuguese uses the preposition a plus the definite article. For Saturday, a plus o makes ao sábado. Even though it is singular, it translates to on Saturdays because this structure implies a weekly habit. You can also use the plural aos sábados for the exact same meaning.