Mense Ianuario aqua in fonte frigidissima est.

They are in the ablative because Latin often uses the ablative of time when to say when something happens.

So:

• mense Ianuario = in the month of January / in January

Both words are ablative singular:

• mensis → mense
• Ianuarius → Ianuario

The adjective-like month name Ianuario agrees with mense in case, number, and gender.

Latin often does not need a preposition for expressions of time when.

So English says:

• in January

but Latin can simply say:

• Ianuario
• or more fully, mense Ianuario

This is normal Latin usage. You may also see in mense Ianuario, but the preposition is often unnecessary.

Aqua is the subject of the sentence, so it is nominative.

The sentence is saying something about the water:

• the water ... is very coldest / coldest / very cold

Since aqua is the thing that is something, it takes the nominative case.

Also notice that the verb is est, which is singular, matching the singular subject aqua.

Because in can take two different cases depending on the meaning:

• in + ablative = in/on a place, showing location
• in + accusative = into/onto a place, showing motion toward it

Here there is no movement. The water is in the spring/fountain, so Latin uses:

• in fonte = in the spring/fountain

If there were motion into it, Latin would use in fontem.

Fonte is the ablative singular of fons, fontis, a third-declension noun.

Basic forms:

• nominative singular: fons
• genitive singular: fontis
• ablative singular: fonte

Because it follows in in the sense of location, the ablative is required:

• in fonte

Because frigidissima must agree with aqua.

Aqua is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• frigidissima

The other forms would not match:

• frigidissimus = masculine nominative singular
• frigidissimum = neuter nominative singular or masculine/neuter accusative singular

Frigidissima is the superlative form of frigidus, meaning cold.

So:

• frigidus = cold
• frigidior = colder
• frigidissimus / frigidissima / frigidissimum = coldest, very cold

In context, frigidissima est can be understood as:

• is coldest
• or sometimes more naturally in English, is very cold

Latin superlatives often overlap with English very + adjective as well as -est forms.

Latin word order is much more flexible than English word order because the endings show each word’s role.

So Latin can place est at the end quite naturally:

• Mense Ianuario aqua in fonte frigidissima est.

This is a very normal Latin pattern. English usually needs a stricter order, but Latin does not.

Putting est at the end can also make the sentence sound neat and balanced.

Yes, often they could.

For example, Latin could also say things like:

• Aqua in fonte mense Ianuario frigidissima est.
• In fonte aqua mense Ianuario frigidissima est.
• Ianuario mense aqua in fonte frigidissima est.

The basic meaning would stay the same because the endings still show the grammar.

However, word order in Latin is not random. Different orders can change:

• emphasis
• style
• rhythm
• what the writer wants to highlight

Because Ianuarius is the name of a month, so modern editions usually capitalize it just as English capitalizes January.

In ancient Roman writing, capitalization worked differently from modern English, so this is mainly a convention of modern printed Latin.

So in a modern textbook, Ianuario with a capital I is normal.