NEW YORK — After a busy holiday weekend in shopping malls, millions of Americans are expected to log on and keep shopping on the day dubbed Cyber Monday. That day, the Monday after Thanksgiving, has been the biggest online shopping day of the year since 2010. The day could take on added importance after a Thanksgiving weekend that saw fewer shoppers and lower spending than last year, according to some estimates. Retailers have been pushing deals all month and particularly the past week, hoping to spur customers to spend.