First year question

I know everyone says you can do whatever you want in university but how hard is it to catch up if I missed this day? Is stuff posted online or is it really just not a big deal? Do profs give all the test dates at the start of the year so I would know what’s happening this day

I know everyone says you can do whatever you want in university but how hard is it to catch up if I missed this day? Is stuff posted online or is it really just not a big deal? Do profs give all the test dates at the start of the year so I would know what’s happening this day