Here are the best fit answers for each numbered blank:

1. **popular**
2. **on**
3. **dress up in costumes**
4. **candy**
5. **carved**
6. **fun**

So, the complete text would read:

Halloween is a traditional celebration that began in Europe hundreds of years ago. Nowadays it is a special day in several countries around the world, but it is perhaps most **popular** in Canada and the United States. It always falls **on** October 31. Around this time, people like to tell scary stories and watch horror movies. In the evening, children wear **dress up in costumes** that make them look like interesting animals such as a dinosaur or an elephant, monsters or people. Children then go from house to house and ask for **candy** by saying, "Trick or treat?" Many older kids and adults also put on costumes and go to Halloween parties. Jack - o' lanterns are **carved** with lights inside and faces cut into them. They are very common during this time and a lot of people like to put them in their windows or just outside their doors. Most people, both young and old, have a lot of **fun** on Halloween.