No, it is not compulsory and it'd be packed like most welcomes.
Add On: Faculty welcomes like Science are packed but if you get in early to grab seats you'll be able to meet people/gain friends. From my experience anyway.
Just keep in mind that usually these welcome days go through a lot of information you can find yourself in the handbook and on the University's website.
Definitely go if you're looking to get a head start on meeting new people though.