For those who didn't managed to find the correct answers, here they are;

1. Which actress made her film debut in Halloween (1978)?

Jamie Lee Curtis has been in seven Halloween movies: Halloween (1978), Halloween II (1981), Halloween H20: 20 Years Later (1998), Halloween: Resurrection (2002), Halloween (2018), Halloween Kills (2021), and Halloween Ends (2022). She played the iconic role of Laurie Strode, the protagonist in most of the films, and even had a cameo as a telephone operator in Halloween III: Season of the Witch.

2. Where does the Ghostbusters (1984) trio find their first ghost?

The Ghostbusters trio finds their first ghost at the New York Public Library. After receiving a report of a ghost, they investigate the library where they find evidence of paranormal activity, and a full-torso apparition is witnessed.
watch it here: https://www.youtube.com/watch?v=qtRJBGyEIQQ

3. What ancient Celtic festival is Halloween said to originate from?

Samhain is a Gaelic festival on 1 November marking the end of the harvest season and beginning of winter or the "darker half" of the year. It is also the Irish and Scottish Gaelic name for November. Celebrations begin on the evening of 31 October, since the Celtic day began and ended at sunset. This is about halfway between the autumnal equinox and winter solstice. It is one of the four Gaelic seasonal festivals along with Imbolc, Bealtaine, and Lughnasa.

Special bonfires were lit, which were deemed to have protective and cleansing powers. Like Bealtaine, Samhain was a threshold festival, when the boundary between this world and the Otherworld blurred, making contact with the aos sí (the 'spirits' or 'fairies') more likely. The souls of dead kin were also thought to revisit their homes seeking hospitality, and a place was set at the table for them during a meal. Mumming and guising were part of the festival from at least the early modern era, whereby people went door-to-door in costume, reciting verses in exchange for food. The costumes may have been a way of imitating and disguising oneself from the aos sí.