When Buffy goes to the mausoleum during school hours in 1.02 The Harvest, Angel appears behind her to try and stop her going into the sewers. It’s the middle of the …

We first learn that vampires can be killed by direct sunlight in 1.02 The Harvest, when Buffy says:
Xander: “So, what else?”
Buffy: “What else what?”
Xander: “For vampire slayage.”
Buffy: “Oh, fire, beheading, sunlight, …