The Greek in Acts 20:7 that is often translated as “on the first day of the week” in English is translated in Matumbi as “on Sunday,” since the week starts on Saturday for most Bantu speakers. (Source: Pioneer Bible Translators, project-specific notes in Paratext)
See also complete verse (Acts 20:7) and Translation commentary on Acts 20:7 for views on what day this refers to.