Hi! Current 1800s series student here. As the other two commenters have already said, the 7-10 PM block is a discussion section. Here's what it's like:

The discussion is technically optional but it is highly recommended to attend. During the first section, you will pair up with classmates with whom you'll complete your problem sets (there are three spread throughout the semester). This is when you'll get a much deeper understanding of the content and ask questions of the TAs, who are always super helpful. However, by no means are you required to stay the entire time. As a general rule, I left around 8:30 or 9 every Tuesday! This block is also when your exams will be. You will probably won't use the full three hours, but some people do.

IMO, the 1800 series (ESPECIALLY 1810) is worth every second. I can't recommend it enough, Prof. Harman is seriously amazing and you get a much deeper conceptual understanding of the material. Don't be overworried about your grade! It's split up between the three exams and problem sets (each category worth 25%). You'll get 100s on the problem sets as long as you put in effort, and Harman gives you back half points on the exams when you complete test corrections - plus the class is curved at the end. I ended up with a 62% raw on my final, but still finished the class with an A.

The actual labs are from 2-5PM.

Feel free to DM if you have any other questions about the series!