Why was "that" used in the beginning of the sentence?
The word "that" here does not actually come at the start of the sentence. Instead, it links to "and so it was decided" right before. "That" just sounds like the start of the sentence because there is a pause in between. So the full sentence here is "and so it was decided that once every year when the weather got chilly...". The word "that" in this case is used as a conjunction to link the two parts of the sentence together.