Southgate showed a rarely-seen ruthless edge as he axed Manchester City winger Grealish and Tottenham playmaker Maddison following their poor form at the end of the domestic season.
The England boss also showed no sentiment by leaving out Manchester United defender Maguire, who has been a mainstay of his reign but was unable to recover from a calf injury in time for the tournament in Germany.
Southgate had been due to submit his final 26-man squad on Friday evening before it was revealed to the nation on Saturday.
But news of his decisions to leave out Grealish, Maguire, Maddison, Curtis Jones, James Trafford, Jarrad Branthwaite and Jarell Quansah leaked early, prompting the Football Association to announce the squad on Thursday instead.
Southgate believes his group now need to come together after an emotional day as they start to focus on their bid to win a first major trophy since the 1966 World Cup.
"I didn't want to keep the players waiting until Friday. I think that would have been a mess, really," Southgate told the BBC.
"It wouldn't have been fair on the players that we've released, wouldn't have been fair on the players that were waiting to have confirmation that they were in.
"Inevitably, we knew when we named a 33 there would be a period of a nice prep camp, and then there was going to be a couple of days where there would be a lot of anxiety and a difficult couple of days for us all.
"But we'll close that down later tonight and now tomorrow everybody's heads are clear."
England play Iceland at Wembley on Friday in their last warm-up before the Euros.
Southgate's men, one of the leading contenders for the trophy, open their Group C fixtures against Serbia on June 16th before facing Denmark and Slovenia.
"When they come back in on Sunday they can really start looking forward to the tournament," Southgate said.