In my humble opinion - and experience - a day 5 test is of more use than Day 1. As Thailand Ryan says, arrivals pitch up here having had a PCR test up to 72 hours before departure, and may well pick up the virus on the journey here (airports, flights etc) but will not be positive on the day 1 PCR test. I was aware of a very mild sore throat on day 3 which alerted me to the possibility of having picked Covid up, tested negative with an Antigen test on that day and then positive on day 5 (both my own Antigen and the hospital day 5 PCR). I was diagnosed as having Omicron, in a rural area that apparently has been the only case in the area, which suggests mine is not a local infection. As you say, the more testing the better.