Day 1 on the itinerary corresponds to the first date showing on the departure. This is shown in bold on the dates and prices panel on the trip page, or if you have already booked, Day 1 is the date that shows on your booking page.
For example on this Adventure Hiking in Slovakia trip departure day 1 is Thursday 01 June 2023.
Dates and prices panel view:
So when looking at the logistics information in the Getting There section:
The trip starts at 17:00 on Thursday 01 June 2023.
You should therefore book a flight to arrive by 16:30 on Thursday 01 June 2023.
When looking at the Itinerary information, Day 1 is Thursday 01 June 2023