Why does 要 mean "will" and not "want" in 这个星期五晚上我们要去酒吧? 这个 星期五 晚上 我们 要 去 酒吧
  
  Zhège Xīngqīwǔ wǎnshang wǒmen yào qù jiǔbā


is translated as:


  We're going to a bar this Friday night.


What makes this future tense vs the verb want?
What makes this future tense vs the verb want? 


If it happened in the past,  the sentence would be "这个 星期五 晚上 我们 去了 酒吧" - 了 indicates the action is completed

"这个 星期五" (this Friday) apparently refers to the up coming Friday


我们 要 去 酒吧 could mean "we need to go to a bar" or  "we want to go a bar" 


We cannot be sure without more context, but with the information of '这个星期五' we can translate 'need to' or 'want to' as 'will'