Learn Italian: Organizziamo una serata speciale per l'anniversario. - We are organizing a special evening for the anniversary.

In Italian, the regular present tense (organizziamo) is often used to mean the English present continuous ("we are organizing"). It is very common to use the simple present to talk about ongoing actions or near-future plans.

What is the difference between sera and serata?

Both translate to "evening", but serata focuses on the duration and the experience of the time spent. Because this sentence is about a planned event with activities, serata is the right word.

Serata is a feminine word, so why doesn't speciale end in an "a"?

Speciale belongs to a group of adjectives that naturally end in -e in the singular. These adjectives use the exact same -e ending for both masculine and feminine nouns. They only change to -i in the plural.

Does per combine with l' in this sentence?

No, unlike prepositions such as a, di, or in, the preposition per does not merge with the definite article to form a combined word. You just write them separately as per l'anniversario.