先日

1. Origins, structure, and basic meaning

• Structure: 先日(せんじつ) refers to a day in the fairly recent past or a recent occasion.
• Basic usage: It expresses thanks for help received with a recent move.
  • 先日は引っ越しを手伝っていただき、ありがとうございました。

2. Common pitfalls and similar expressions

• The day before yesterday and a recent day
  • 一昨日(おととい) identifies the day two days ago. 先日 refers back to a recent event without specifying the date and suits polite speech or writing.