Omitting と when listing things


So I was wondering if someone could clear up when you are allowed to omit と
when listing things and when it's usually recommended to omit と.

Here are some examples of sentences I found in a book that either omit と
altogether or only omit one or two and it's confusing.

> 今回の旅行ではスペイン、イタリアそしてフランスと、おもに南ヨーロッパを中心に回った。  
>  On this trip I traveled mainly around southern Europe, to Spain, Italy, and
> France

Why in the above sentence do they omit と after スペイン and イタリア but include it
after フランス?

Then in the following sentence they omit と altogether:

> 夏休みにタイ、マレーシアそれからインドネシアの三カ国を回ってきた。  
>  During the summer vacation I traveled around three countries: Thailand,
> Malaysia, and Indonesia.

Then in the next one they include と between the first and second noun but
leave it between the second and third noun:

> 初級のクラスは月曜日と水曜日、それから土曜日にやっています。  
>  We have a beginners' class on Mondays, Wednesdays, and Saturdays.

Does this mean that you can choose to use と or leave it out when you like?




I tried to investigate a bit (and was surprised by the number of white papers
about the classification of conjunctions in the Japanese language).

I found that this discussion seemed very close to your question:  
<https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1380980358>

A summary of the replies in that discussion:

  * In enumerations of more than 2 things, you usually use the conjunction/particle と only once ; using more than that is an influence of the usage of "and" in English, and might also sound childish

  * It is OK to use と after the first item in the list ; or before the last one. However there are cases where the と only "applies" to the items just before and after it (grouping them in a same category). (Maybe imagine a sentence like "the boat contained men and women, apples, and bricks" where the first 2 items would be grouped in a same category by "and").

You might also be interested in this discussion a bit similar but about や,
where it is also mentioned that its position in the enumeration connotes
different groupings:  
<https://oshiete.goo.ne.jp/qa/3042585.html>

