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as far my knowledge goes, it isnt mandatory. but the college recommends u to attend it only to interact with fellow acquaintances and socialize, also to show u arnd campus.
but there could be exceptions.
 
Last year, I missed my orientation week because of my visa and there wasn't any problem with the university. Although its very tough to interact with people afterwards as everyone had already made their own groups of friends, so it took a while for me to fuse in with them.