Arsenal host Leicester City on Friday night, marking the first time in league history the season's first match won't be played on a Saturday.
Two minutes later, the comeback was complete, with super-sub Giroud sending the home fans wild via magnificent header from a Xhaka corner, despite being held by Wes Morgan.
"Obviously I knew that I was going to have a chance so we tried to push", Giroud said.
The France global, who moved to the Gunners for a club-record deal this summer, scored 28 goals from 2,408 minutes of action in Ligue 1 last season.
Arsenal won their first match of the season with a 4-3 thriller against Leicester City.
The substitutions nearly paid off as Ramsey was so close to restoring parity for the Gunners with his first touch, heading wide Hector Bellerin's cross from the left flank in the 68th minute.
Less than three minutes after going ahead, their good work was undone, as the Gunners were caught off guard at a set piece.
Shinji Okazaki cancelled out Lacazette's opener with a close-range header after Maguire had been left unmarked to nod Marc Albrighton's deep cross back across goal.
Slater: Manchester duo poised to challenge Chelsea for title
Arsenal manager Arsene Wenger has ruled out Leicester City midfielder Riyad Mahrez moving to The Emirates this summer. The pressure is most definitely on for the red half of north London after missing out on the Champions League.
During a wide-ranging panel discussion, Wenger took questions from the audience and was asked directly if the negativity from some sections of Arsenal fans over his future had filtered through to the team's performances on the pitch.
Lacazette, a club-record £46.5 million ($60.3 million, 51.1 million euros) signing from Lyon, made a dream start to his home debut by heading Arsenal in front with just 85 seconds on the clock.
Arsenal looked a beaten team when Leicester led 3-2 early in the second half after Vardy grabbed his second goal before the substitutions gave them new vigour in attack.
"The Premier League is the most competitive league in the world and Arsenal are one of the best clubs in it, year after year they've shown their consistency", he said.
Schmeichel thwarted Oxlade-Chamberlain and Bellerin before Wenger turned to his bench in the 67th minute, pitching Ramsey and Giroud into the fray in place of Elneny and Rob Holding.
It seemed an unlikely platform for victory, but Ramsey neatly controlled Xhaka's dinked pass to drill in an equaliser and moments later Giroud's header from Xhaka's corner completed a remarkable turnaround.
Here are four talking points from the game.
"Insecurity goes quickly through a team and we had many experienced defenders out tonight - Koscielny, Mustafi, Mertesacker".