Day.js is a lightweight JavaScript library designed for parsing, validating, manipulating, and formatting dates. It offers a simple API that is easy to use, making it a popular choice for developers who need to handle date and time in their applications without the overhead of larger libraries. One of the key features of Day.js is its compatibility with the Moment.js library, allowing developers to transition smoothly between the two. With a size of only about 2KB, Day.js is efficient and fast, making it suitable for both small projects and larger applications that require date manipulation.