What does 则 mean in ...而今年的9月20日则是安室奈美惠40岁的生日?
则 here, as an adverb, means 'yet'. The structure of 而...则... could be interpreted as 'while'.

So, "而今年的9月20日则是安室奈美惠40岁的生日。" is "while Sep. 20th this year will be the 40th birthday of Anshinaimeihui's."