Not just one night, sometimes three. It was actually considered a period of 'no time'. The days weren't named during 'no time' and the rules were different, hence why the ancestors could temporarily rejoin the living.
The precise duration depended on how long the Druí felt they needed to adjust the calendar. Or so some think who've considered this from the perspective of an agrarian culture using astronomy to track time and having to make periodic adjustments to their calendar.
The single night of All Hallows Eve, (which I've always spelt as Hallowe'en and that's what we were taught in school), is a later Christian overstamp (like Christmas which just missed). The lights in the window, dressing as the sí and leading them to honey and 'treats' are relics of the older traditions though. I guess the Christian monks didn't feel they needed the extra days because they were using a different calendar system and just the one night of the walking dead was pagan enough for them.
Also, Samhain was an important festival, being the climax of the harvest, and therefore, of the agricultural year. It was, like many festival appropriated by the church who, because of the association with the departed, made it all souls day. When the Spanish colonised America they changed the date of the 'Day of the Dead' to match - so it's a weird bit of second hand Celtic imperialism - but I guess they got us back with the potato.
The mainstream Catholic and Protestant and Atheist Irish couldn't care less. There are some more hard line Christians in Ireland like elsewhere who take a dim view, but they tend to think of anything except what they believe in as Satanism in one form or another.
Neopagans in Ireland have mixed views. If there's 'appropriation' going on, and people are 'Lady Gregory-ing' the hell out of whacky interpretations of what might have been celebrated once at these festivals, then yes there's offence taken to that. But otherwise, no. People are free to re-interpret these festivals as it seems fit to them, because until the equivalent of The Dead Sea Scrolls for pre-Christian Ireland turns up, it's all a matter of interpretation and debate as to what went on. Very serious academic work is however untaken in Ireland to this end. Check out Cork University's digital archive.
The Irish basically are very laid back about people copying them. Tap dancing is copied from Irish dancing. Rhyming as we know it today was basically invented in Ireland, the ancient greeks, ancient latin none of them rhymed till the Irish monks brought it across Europe. You have a surname? thats Irish culture, Irish invented the hereditary surname as we know it. The O' means "son of", What was the Surname of Jesus? He didnt have one, but the Irish at his time of Birth did. The Norse directly copied this system "Ericsson", they just added the "son" at the end, Erics son (O'Eric). Before they got to Ireland they were called things like "Eirc the tall", "Eric the blonde". In UK it would "Paul the Tailor", "Carl the smith". If Carl the smith had a child, the child wasnt called "frank the smith", it was reserved for black smiths, not until they copied Irish naming culture did it get passed down. As for Hallowe'en, well Irish people dont care, in fact some copy the Pumpkin carving cause its cool, originally that was turnip carving but the Pumpkin ones are cool.
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Many Latin American countries celebrate the days of the dead (November 1st and 2nd) At least in Mexico, Halloween has been added to this, making it a three day celebration instead of a two day celebration. In some places in Mexico, it is as important a holiday as Christmas, with people putting up altars to their dead relatives, decorating graveyards and having a meal on the grave, making sand paintings and having parties in the villages where there are multiple bands playing, people in groups dressed in similar costumes or as katrinas (dressed up skeletons), and food. They believe their relatives can come through at this time also.