According to your textbook saying feb-u-ary instead of february is an error in

The answer is pronunciation

Pronounciation of a word will include the intonation, the stress, the articulation, and how we produce the sound for that word.

Good pronunciation will conveys the perception of well-educated person by your listener and will make you seem more trustworthy.

This is a pronunciation error. These types of errors are often due to changes in the way that words are spoken over time. In addition, dialects of the language can lead to these types of errors, which are often not intentional. The errors simply come about because of how the language (and how we process it) changes over the years.