how do Professors, faculty and postdocs go about attending conferences during the semester/academic year? I understand there are conferences during the summer and winter break, what about conferences during the semester? Do professors habitually cancel their classes?
The problems that can arise involve teaching schedules and lab management. The latter can usually be handled for some days by delegating to subordinates.
For teaching, however, there are a couple of strategies and they might differ depending on the course level. Conferences are seldom (in my field, anyway) as long as a week and often include week ends - non teaching days. Sometimes you can just give students a set of tasks to complete in your absence and catch up with them when you return - readings, projects, writing, whatever. If they miss a lecture or two it isn't normally a huge issue if the material is available to them in other formats.
But, we also tend to cover lectures for our colleagues when they need to be away. TAs can also do this to some extent. Having an occasional "work session" rather than a lecture might be a good thing. A substitute can answer questions if they are in the same field and familiar with the course. Some will even schedule an exam that is proctored by a substitute.
One way to think of it is that learning is something the students do and it is much more important what they do than what the professor does. If the instructor can make those activities continue in their absence then feeding out new information in a lecture may be unneeded.