Go to the master timetable
https://adweb.cis.mcmaster.ca /mtt/
Figure out the times when all of your courses are offered, and which sections you'd like to be in.
Go onto SOLAR (the same way you did when you picked your courses), select 'Modify' next to the course you want to change, and select the sections you want from the drop-down menu. By this point, many sections are full, so there's no guarantee you'll get the ones you want, but it can't hurt to try.
Once you've made your changes for the course, click the 'modify' button. Then repeat that process to make the changes for all of the courses (you can only do one course at a time).
Verify and apply the changes. It will take a little while for your timetable to reflect the changes you made.
There's no guarantee you'll be able to have wednesdays off, though--sections will be full or they'll conflict with other sections or they'll only be offered on wednesdays. It's possible, depending on the scheduling (and what faculty you're in), but it's not guaranteed.