For the ancient Celts who lived in Ireland, Samhain was the most important time of the year. The harvest was in, the dark months were beginning, and they believed it was a night when the line between our world and the 'otherworld' was thin. They lit big bonfires and gathered together.
People dressed up in costumes made of straw and old animal skins. The idea was that any spirits wandering around wouldn't recognise them - they'd just look like other spirits. That is where dressing up for Halloween started.
Before pumpkins were used for jack-o'-lanterns, Irish people carved scary faces into turnips. They put a candle inside and set them in their windows. (Turnips are much harder to carve than pumpkins, so this took real effort.) When Irish families moved to America, they discovered pumpkins - much bigger, much softer - and the jack-o'-lantern as we know it was born.
Other Halloween traditions came from Ireland too: bobbing for apples, telling ghost stories, and going from house to house collecting treats (called 'guising' in Ireland - children would sing a song or tell a joke before getting a treat). It is amazing to think that the night you celebrate every October started here, over 2,000 years ago.