Day 1 on the itinerary is the first departure date of your trip. If you have not yet booked, Day 1 is displayed in bold in the dates and prices panel on the trip page. If you have already booked, Day 1 is the start date shown on your booking page.
For example on this Adventure Hiking in Slovakia trip, the screenshots below show an example where departure Day 1 falls on a Thursday.
Dates and prices panel view:
Booking view:
So when looking at the logistics information in the Getting There section:
In this example, the trip starts at 17:00 on Day 1 (Thursday).
You should therefore book a flight to arrive by 16:30 on Day 1.
When looking at the Itinerary information, Day 1 is the date shown in bold on your departure — Thursday in this example
