Essentially, if you use Day 4 to work on a phonemic awareness skill such as syllables with your big book or poem, then I would focus on that skill every week till they had it and periodically keep reviewing it as necessary.
Technically speaking, I think it's almost impossible to teach only one thing at a time so the only time I would focus on only one concept is when I introduce it. After that it would be a review along with other things that they're learning. It would probably depend on the students and the concept taught. And, teaching opportunities arise all the time as teachers work with students. Also, if you're focusing on phonemic awareness, you're targeting several phonemic awareness skills just by immersing students in language. You're saying sounds slowly in words in writing but you could be working on syllables with your poem/big book on the day you focus on phonemic awareness.
Sometimes rhyming, or some other early literacy skill, will take a while for some kids to develop. If they seem to simply need more exposure to language, such as when they're not hearing rhymes, teach another skill and then come back to rhyming.