[Ask the Rabbi] Ep7 - The 4 Rosh Hashanahs 

The Mishnah describes four separate starts of a year, or Rosh Hashanahs, but nowadays we only mark two of them: the 1st of Tishrei which we all know as Rosh Hashanah, and Tu Bishvat - the new year for trees.
Rabbi Elton explains why only these two have remained with us, and why it might be more exceptional that Tu Bishvat 𝗜𝗦 marked rather than that the others are 𝗡𝗢𝗧.