What is meaning of "no holiday undertaking"?

‘The planting of new nation in America was no holiday undertaking.’ The sentence is taken from a book on US history.What is the meaning of ‘holiday undertaking’ in this sentence? Moreover, is holiday undertaking a phrase, idiom or metaphor?

On holidays you usually do things that don’t require serious efforts. “No holiday undertaking” might mean the opposite situation. It is not an easy undertaking and requires a lot of serious efforts.

If the theme of the article is not about American holidays, the expression is a metaphor. I don’t know what you want to mean by “idiom”.

There is an expression “朝飯前” in Japanese, which literally means tasks that could be done before breakfast. This seems to be an idiomatic expression, which means “easy”.

Would you tell me the difference between metaphors and idioms?

Thanks Yutaka. Actually i could extract the meaning of “no holiday undertaking” from the sentence above that it was not an easy task but i wanted a native speaker to explain it a bit more.Anyhow, idiom is a group of words (phrase) which actual meaning can’t be derived from individual words used in it, for example the idiom “a load of cobblers” doesn’t mean ‘a lot of shoe makers’ but it means ‘nonsense’.On the other hand metaphor is figure of speech that describes an object or action in a way that isn’t literally true, but helps explain an idea or make a comparison.For example in our native language we use the word moon for beauty of some one.We say "my moon has reached."It doesn’t mean that she/he is actually moon but we use the word moon as a metaphor for the beauty because in our country moon in considered a beautiful entity.

“Actually i could extract the meaning of ‘no holiday undertaking’ from the sentence above …”

I imagined so. If you couldn’t have understood the meaning of the phrase “no holiday undertaking”, you wouldn’t have used the word “metaphor”. Thank you for showing me the definition of “idiom”. Am I right in thinking that some metaphors might become set phrases and loose the original “wit” shown in them? I wonder if they are called “idioms”.

white elephant
Where under the sun has he gone?
There’s no new thing under the sun.

Can you provide some examples of those metaphors please?

In the above text white elephant ,under the sun and nothing under the sun are clearly idioms as their literal meanings has nothing to do with their implied meanings.White elephant when used in a sentence as idiom it doesn’t mean an elephant of white color, rather it will mean something which is expensive for some one to keep.In the same way ‘under the sun’ in above sentence doesn’t mean below sun but it means ‘on the earth’ to native speakers.Similarly, in sentence ‘it is raining cats and dogs outside’ , cats and dogs has nothing to do with the intensity of raining.The sentence simply means ‘it is raining heavily outside’.
On the other hand in metaphors ‘curtain of night’ ,‘sea of grief’ night and grief are used as metaphors for the covering property of curtain and vastness of sea respectively.