What kind of periods can I put before にわたって?

Question

I came across this question when doing some JLPT N2 practice.

（　）にわたってこの町には観光客が大勢きた。

a 五月

b 五月の連休

c 一週間

I think that all three options are correct syntactically. All three are periods of time, so I thought I should choose the semantically best answer - b. This is because the other two options are not as worth talking about. I mean, "consecutive holidays" sounds more like a topic to chat about/discuss than just "May" or "a week". b also makes sense because during consecutive holidays, people do tend to travel a lot, so it would be natural for a town to have a lot of visitors during that time.

However, the correct answer is c and I don't see why. My book says this about the use of にわたって:

場所・時間・回数。範囲など、幅が大きいことを表す語につく。

So I should put a long period of time (幅が大きい) before にわたって. However, the correct answer is just a period of one week. And speaking of 五月の連休, I can only think of Golden Week, so my answer also is a period of one week.

Why is the correct answer c and why are the other answers wrong?

Answer 1

The only corrrect (or natural-sounding) answer would be c) 一週間{いっしゅうかん}.

To use 「～～にわたって」 the way native speakers would feel most appropriate, the ~~ part must physically be in the following structure:

「Cardinal Number + Counter Word of Time period + （間{かん}）」

Among the three choices, only 「一週間」 fits that deccription.

「五月{ごがつ}」 does not fit because strictly speaking, the 「五」 here is the ordinal number in nature ("the fifth month") if not in the physical form. You could use 「一カ月（間）」 or 「五月の一カ月（間）」 instead. 「31日間{にちかん}」 and 「約{やく}30日間」 would also be appropriate.

「五月の連休{れんきゅう}」 could not be used, either, as 「連休」 does not refer to a specific time length. Besides, how many consecutive holidays there are in May (the Golden Week) differ from one year to the next depending on what days of the week the holidays fall on.

You can safely say 「五月の連休[中]{ちゅう}に」 or simply 「ゴールデンウィーク中に」 instead without using わたって.

Exceptions:

It is also appropriate to use the following phrases in 「[Phrase] + わたって」 without using an actual number.

・「長{なが}き/長期間{ちょうきかん}にわたって」

・「長い年月{ねんげつ}/月日{つきひ}にわたって」