the national day

Inconsistent or eccentric style is common in Wikipedia, which anyone can contribute to or edit. I have added a "The" in the place you indicated. I have not corrected another problem in the Wikipedia entry, namely the persistent capitalisation of "National Day", "Public Holiday" etc when used in a generic or descriptive sense.

You would use "the" if you were talking about a national day in a generic sense, and you would not capitalise, e.g. "In France the national day is July 14th", but if the day is actually called "National Day" (as it may be in some countries including Singapore) then there would be no definite article ("the") and the two words would be capitalised. ("What will you do on National Day"). The list in the Wiki shows that most countries' national days are not called "National Day".

Thank you, C.

From Wiki:

-Table tennis is governed by the worldwide organization International Table Tennis Federation (ITTF), founded in 1926. ITTF currently includes 218 member associations.

Is this also an example of Wiki's being guilty of using THE in a wrong way and we must add THE before the last ITTF?

In my opinion, yes to both questions. Wikpedia consists of material from all kinds of people, with varying levels of competency in English.

Today is China's national day. My daughter, who is studying at Xiamen University, has the rest of the week off.

From Wiki:
Francis George Fowler (1871–1918), familiarly known as F.G. Fowler, was an English writer on English language, grammar and usage.

To refer to a language nowadays, we usually say English or THE English language; is THE in that Wiki entry omitted because of parallelism?

He was a writer on the English language. He wrote on English language, grammar and usage.
In the second sentence, "language, grammar and usage" are grouped together and so adding "the" before "English" would be awkward and unnecessary.
In the first sentence it effectively acts as part of the noun, and in the second, as an adjective.

Thank you~