What: When Hydrogen plasma emerged, and a few minutes later when protons and nuetrons started forming the nuclei of Hydrogen, Helium, and Lithium (Fun Fact: this is part of proof that the Big Bang happened, because these were the elements that the Universe started from, and currently 99% of observable matter in the Universe has been identified as either Hydrogen or Helium)
When: 1 second after Big Bang, 3 minutes for the nuclei to start forming