What is an enumeration? Without article it is a short compact formula and no less understandable.

How are smokers putting on weight bad for health?

At http://www.grammaring.org/. The

Gerund is a word. com/the-gerund-as-subject. The same org for. com/by. eu/the-gerund.

Simply because it is shorter, because the article does not achieve much, and because without article the sense is as clear as with article.

The preposition on is used in connection with “day”,”on this day”, parallel to German “an diesem Tag”. On is used frequently, but often for the days of the week, which have the word day: Monday, Tuesday, and weekdays. Some people use the word “on” when a day is connected with a “morning”: a Monday morning, a Saturday night. On a Monday or a Thursday will be used. It’s the most widespread used name in English language.

When “morning, afternoon,” is used alone, then the concept of period of time prevails and “in” is used: in the morning, in the afternoon

“nights” can be used with a lot of prepositions, at night, by night, during the night.