Learn Portuguese (Portugal): Janeiro é um mês muito frio.

Normally, no. In Portuguese, months like janeiro are written with a lowercase letter. It is only capitalized in this sentence because it is the very first word.

Why do we use é instead of está for the weather here?

We usually use the verb estar to talk about how the weather feels right now (like temporary conditions). However, this sentence is stating a general, permanent fact about the month of January as a whole. When describing an essential or permanent characteristic, we use the verb ser, which gives us é.

Why does frio come at the end of the sentence?

In Portuguese, adjectives (describing words like frio) usually come after the noun they describe (mês). So, instead of saying "a very cold month" like in English, the word order is literally "a month very cold" (um mês muito frio).

How do we know the word mês is masculine?

You can tell it is masculine because it uses the masculine word for "a" (um) and the masculine form of the adjective for cold (frio). If the noun were feminine, those surrounding words would have to change to uma and fria.