You'll gain the most from your course if you attend every session as missing one can make it hard to take part fully later. Course outlines tell you what you can expect to achieve – this assumes that you are present throughout. However, we appreciate that absence is sometimes unavoidable!
If you know that you'll be missing a session, you may want to contact the department running your course so that we can let your tutor know to forward you any catch up work or reading so that you're prepared for the next class.
Some courses (usually accredited courses) may also have a minimum attendance requirement – please check your course outline or your course documentation.