Complex sentence: An Open Day is held annually in October, where visitors may listen to talks about astronomy, meet astronomers and tour inside many of the telescope domes which are open to the public on this one day of the year.
Split into simpler sentences: An Open Day is held annually in October, where visitors can enjoy many activities on site. They can listen to talks about astronomy, meet astronomers and tour inside many of the telescope domes which are open to the public on this one day of the year.