From the rules:
When the last Support Token in the currently active
Period Column is taken, the game is immediately paused
and two changes take effect:
1. The Abolitionist Card Deck in the currently active
Period Column is removed from the game. Any of
the current Period’s Abolitionist Cards already in the
Abolitionist Queue remain where they are. Any new
cards will be drawn from the next Period’s Abolitionist
2. The next Period Column is activated and its tokens are
now immediately available to be taken by all players.
The last support token being bought is what triggers the next period column, other tokens don't matter.