With a semi-final spot still up for grabs, England need to ensure a bonus point victory to stand any chance of progressing.
Day will captain a team that shows 11 changes from the group which lost to South Africa last Sunday. All three of his Saints Senior Academy clubmates are in the squad, with centre Tom Stephenson and fly half Will Hooley both making their first starts of the tournament and prop Danny Hobbs-Awoyemi lining up on the replacements' bench.
But all attention will be on Day, who is honoured to have been given the responsibility.
"It's a massive honour to named captain for this game," he said. "It's something that you dream of when you're little and to achieve it is pretty exciting - I'm massively proud and I just want to go out there and do a good job.
"We're staying in the same hotel as the USA and we know that they're going to be up for it, but we've prepared well and we're looking forward to the game."
Thursday's game is being shown live on Sky Sports from 5:45pm.
ENGLAND UNDER-20 TEAM TO PLAY USA
IRB Junior World Championships
Stade Henri Desgrange, La Roche-sur-Yon
Thursday, July 13th, 2013
Kick off 5:45pm BST
15 Ollie Devoto (Bath)
14 Ben Howard (Worcester Warriors)
13 Tom Stephenson (Northampton Saints)
12 Harry Sloan (Harlequins)
11 Henry Purdy (Leicester Tigers)
10 Will Hooley (Northampton Saints)
9 Alex Day (Northampton Saints)(capt)
1 Alec Hepburn (London Wasps)
2 Scott Spurling (Saracens)
3 Tom Smallbone (London Irish)
4 Harry Wells (Leicester Tigers)
5 Tom Price (Leicester Tigers)
6 Ross Moriarty (Gloucester)
7 Joel Conlon (Exeter Chiefs)
8 David Sisi (Bath)
16 Nathan Morris (London Wasps)
17 Danny Hobbs-Awoyemi (Northampton Saints)
18 Scott Wilson (Newcastle Falcons)
19 Dominic Barrow (Leeds Carnegie)
20 Jack Clifford (Harlequins)
21 Callum Braley (Bristol)
22 Henry Slade (Exeter Chiefs)