Learn Russian: Это середина дня. - This is the middle of the day.

Дня is the Genitive case form of день. In Russian, the Genitive case is used to show possession or relation, much like the English word of. So, середина дня translates to the middle of the day. You might also notice that the е in день disappears when the ending is added—this is a common feature in Russian called a fleeting vowel.

Why do we use это (neuter) when the next word, середина, is feminine?

When you use это to mean This is... to introduce something, it stays in its fixed, neuter form regardless of the gender of the noun that follows. You only change it to match the feminine gender (like эта) if you are saying this specific noun, such as эта середина (this middle).

Where is the word for is in this sentence?

In Russian, the present tense of the verb to be (am/is/are) is almost always left out. The sentence literally reads This [is] middle of-day.

How would I say in the middle of the day to talk about when something happened?

To say something happens in the middle of the day, you use the preposition в (in) and change середина to the Prepositional case: в середине дня. For example, you could say: Вчера в середине дня я был дома (Yesterday in the middle of the day I was at home).