Many Students don't understand different of 一月 (Yī yuè) and 一个月 (yīgè yuè)， if you know the definition of measure word then you should know that the measure word 个 is used in between a number and a noun, so it indicates quantity of noun. so 一个月 means one month, 一月 January same as 五月 means "may" but 五个月 means 5 months.
EXAMPLE :
January is the first month of the year.
一月是一年之中的头一个月。
Yī yuè shì yī nián zhī zhōng de tóu yīgè yuè.
I have your letter of january 2nd .
我收到你一月二日的来信。
Wǒ shōu dào nǐ yī yuè èr rì de láixìn.
She has gone six months with pregnancy.
她怀孕六个月了。
Tā huáiyùn liù gè yuèle.
We went camping in August.
我们八月去了露营。
Wǒmen bā yuè qùle lùyíng.
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