Learn Finnish: Keskiviikkona ei sada. - It will not rain on Wednesday.

The -na ending is the essive case, which is used with days of the week to mean "on" a certain day. By adding -na to keskiviikko (Wednesday), it becomes keskiviikkona (on Wednesday).

Where is the word for "it" in this sentence?

Finnish doesn't use a "dummy" subject like "it" for weather expressions. While in English we have to say "It rains", in Finnish you just use the verb on its own. Therefore, "It will not rain" is simply ei sada.

The verb for "to rain" is sataa, so why is it sada here?

When you use the negative word ei, the main verb must change into its negative form (called the connegative). For the verb sataa, you drop the final -a. Additionally, a spelling rule called consonant gradation changes the strong -t- in the middle of the word into a weak -d-. This gives us ei sada.

The English translation says "will not rain", but aren't these present tense verbs?

Yes, they are! Finnish does not have a separate future tense. You use the present tense for both things happening right now and things that will happen in the future. The context of the sentence (talking about Wednesday) makes it clear that this is about the future.