Auld Lang Syne is a traditional Scottish song that is often played at the end of events (ceilidhs in particular). It is also a song in Catalan, known as L'hora dels adéus. In both languages it is a solemn and rousing song of farewells and times gone by.
In many countries it is also sang at midnight at the start of the year, the 1st of January.
If you are playing at a ceilidh, you may wish to play it twice slow, and then play the B part twice fast.