I had the same reaction as you, so I googled it. According to Wikipedia[1] the eruption did indeed take two days, with shocks starting in the morning and ash starting to fall in earnest at 1 p.m., with "pyrocastic flows" beginning in the middle of the night. People were able to escape and be rescued during the afternoon.

[1] https://en.wikipedia.org/wiki/Eruption_of_Mount_Vesuvius_in_...