I can't speak for other countries, but in American municipalities, garbage trucks typically go from house to house and pick up trash that's been put into cans and placed in the street by the residents. To make things organized, the trash collection companies focus on a different neighborhood every day in a week, returning to that neighborhood on the same day every week or two, depending on the size of that neighborhood. This defines "garbage day" for that neighborhood.

For me, garbage day is every Thursday in my neighborhood, while in the neighborhood next to mine it might be a different day of the week.