hello antman, the word "by" here act as a preposition which means "at some time before or before the end of a given time interval." so, if for instance you promised to "submit the report by Friday", you are committed to hand it over on the exact day (Friday) or days before. sometimes, you'll likely to hear the term "not later than" which implies the same meaning. :)

Hello Antman, If something happens by a particular time, it happens at or before that time. "By eight o'clock he had arrived at my hotel." It means at 8 o'clock or before 8 o'clock. "by Friday" means on Friday or before it. " no later than the beginning of Friday" refers to something happening before Friday and at the latest on the beginning of Friday, in the morning. "no later than the end of Friday" refers to something happening before Friday, at the beginning of Friday or at the latest at the end of Friday ,at night.