Black Friday has become something of an anachronism in the e-commerce era. The day after Thanksgiving marks the official start of the holiday shopping season, but retailers already have spent weeks flooding their websites and customers’ email inboxes with early Black Friday offers. If history is any guide and advertisers are to be believed, the best bargains still are to be had from Black Friday through Cyber Monday, a four-day period known in the retail world as BFCM for short.