The last time this happened was almost 35 years ago. In May of 1983 there were 4 intra-top ten fights: Holmes-Witherspoon, Dokes-Weaver II, Page-Snipes, and Berbick-Gordon.
This month we have Joshua-Parker, Wilder-Ortiz, and Whyte-Browne, plus Povetkin adds a 7th highly-ranked fighter. Of course with most of the action at the end of the month, there's still time for disappointing cancelled/postponed fights. Trying to not be pessimistic.
For those interested, link below shows my current rankings and those post-May of 1983:
http://www.sports-ratings.com/fights/20 ... -1983.html