Popular market is defined as one with 100+ traders or one of the top 5 markets on the homepage at any time. A market is "in" the month that it was created in.
List of my previous markets with 100+ traders:
January: https://manifold.markets/realDonaldTrump/how-will-the-us-take-greenland
December: NONE
November: https://manifold.markets/realDonaldTrump/nyc-mayoral-election-prop-bets
October: BANNED
September: https://manifold.markets/realDonaldTrump/short-fuse-trump-announcement-prop and https://manifold.markets/realDonaldTrump/tyler-robinson-sentence
August: https://manifold.markets/realDonaldTrump/free-mana-can-manifold-change-ais-o
July: https://manifold.markets/realDonaldTrump/does-ai-love-trump-read-description
June: https://manifold.markets/realDonaldTrump/cuomo-runs-independent-after-mayora
May: JOINED MANIFOLD