On Day 26, you will begin answering questions from the OG.

The reason for this is that sentence correction questions typically test more than one concept. Sure, the original sentence might have just one problem (e.g., no subject-verb agreement), but the answer choices will typically feature a bunch of other problems (e.g., verb tense, idioms, faulty comparisons, etc.)
Given all of this, it doesn't make any sense to start practicing sentence correction questions until you have covered all of the content first. So, in the Study Guide, students learn the concepts on Days 23 - 25, and then answer practice questions on Days 26 - 28.

Cheers,
Brent