If we don't use hari before senin and rabu, why do we use it at the beginning for selesa?
I guess "hari" before Senin and Rabu is omitted to avoid repetition.
why is "serta" not correct, instead of "dan"
Serta is most commonly used for a second "and" in a sentence to avoid confusion or to emphasize. Ani dan Budi serta Tono pergi ke pasar. Saya pergi ke bank, toko buku, serta pasar.
So if only one and exist, use dan
Is Hari really necessary?
In my assumption, "hari" is used in formal speech on saying the day today, or it is also used in informal, however, it may be omitted to reduce having to repeat "hari" every time in a sentence.
Why ada and not adalah?
Use ada for something that means "to exist"