No concrete date was ever given, because no one knew when the players demands would be met.
By the way, there's a little bit of precedent here. On several occasions a strike actually happened, and in 1987 the NFL played 3 weeks with replacement players.
The NFLPA went on strike for a month in 1987 upon the expiration of
the 1982 CBA; the league's free-agent policy was the major matter in
dispute. This time, however, the strike only canceled one week of the
season. For three weeks, the NFL staged games with hastily assembled
replacement teams, made up principally of players cut during training
camp and players left out of work from the closure of the United
States Football League two years prior (along with, to a lesser
extent, the Montreal Alouettes of the CFL, who had folded just three
months prior to the strike). They were joined by a few veterans who
crossed the picket lines, including New York Jets defensive end Mark
Gastineau, Dallas Cowboys defensive tackle Randy White, San Francisco
49ers quarterback Joe Montana, New England Patriots quarterback Doug
Flutie, and Seattle Seahawks wide receiver Steve Largent.
Some of the replacement players from that season were (no BS) Death Row Records mogul Suge Knight and Saints coach Sean Payton. Oilers replacement linebacker Eugene Seale stuck around long enough to be selected All-Pro on Special Teams the following year, and Saints replacement QB John Fourcade managed to keep himself in the league for 3 years following the strike.
BTW, regarding the movie The Replacements...
The final replacement game was a Monday Night Football matchup on
October 19, 1987 between the Washington Redskins at the Dallas
Cowboys. The Redskins were the lone NFL team not to have any players
cross the picket line and were surprising 13-7 victors over the
Cowboys who had plenty of big name players cross the picket line. This
game was the inspiration behind the Gene Hackman and Keanu Reeves 2000
movie The Replacements.