In fact, make that three names. But we'll start with what's probably the most common way to refer to today, or specifically, to this evening: uppesittarkväll.
Uppesittarkväll can be translated as 'sitting-up evening' which captures the sense of anticipation ahead of the big event. This is because Swedes have their main Christmas celebration on December 24th, Christmas Eve in English or julafton. So tonight, youngsters might be unable to sleep, while adults are usually busy with the preparations of food and decorations.
Uppesittarkväll is also the name given to a range of special TV and radio programmes on the 23rd, which are shown on several channels and serve as background entertainment to the festive preparations.
Watching these is a good initiation to Swedish Christmas traditions: guests discuss tips for a perfect celebration, and often write rhyming verses for gift tags (an old Swedish custom – the rhyme should give a clue as to the present inside) alongside musical performances and competitions.
You can use uppesittarkväll to refer to these shows, or to the evening in general, whether you're spending it alone or with family.
As for the other two names for December 23rd, in the southern region of Skåne, today is known as lillejulafton (literally 'little Christmas Eve).
Elsewhere and especially among the older generations, you'll hear the 23rd called dan före dopparedan, which means 'the day before dipping day'. Dopparedan or 'dipping day' is an alternative name for Christmas Eve thanks to an old custom of eating dopp i grytan (literally 'dip in stew' referring to flatbread or rye bread dipped in a meaty broth) as part of the festive evening meal. But neither of these truly captures the magic and excitement in the same way as uppesittarkväll.
Ska vi titta på uppesittarkväll på tv?
Shall we watch the December 23rd show on TV?
Hon har bjudit in hennes vänner till en uppesittarkväll
She has invited her friends round on the evening of December 23rd