JUPITER, Fla. — Monday is judgment day. At least, from one point of view.

Major League Baseball’s regular season is scheduled to start March 31. But after making little progress toward a new labor deal with the players’ union, the league doubled down on its position with a threat last week, and that start date appears to be in peril.

M.L.B., which locked out the players on Dec. 2, told the union on Wednesday that it was serious about its self-imposed deadline, Monday, for reaching a new collective bargaining agreement in order to begin the 162-game season as scheduled. If that deadline was not met, the league said, it would begin canceling games, not pay players for those missed games and not reschedule them.

So the baseball world, already frozen, waits. Will the 11th hour spur compromise, or will the sides remain entrenched? Will M.L.B. follow through on its ultimatum, or could the league’s stance soften in the final hours before the deadline?

While it has come entirely during the off-season, this lockout qualifies as the second-longest work stoppage in baseball history. The longest was the 1994-95 players’ strike, which resulted in the loss of more than 900 games and canceled the 1994 World Series.

Although the union objected to M.L.B.’s stated deadline, it made its largest set of moves to date in these negotiations on Saturday, the day spring training games had been scheduled to begin. The union significantly lowered its asks on expanding salary arbitration eligibility and changes to the revenue sharing system, and made tweaks in the league’s direction on the luxury tax. Players considered it a good-faith offer.

But M.L.B., which has insisted that salary arbitration and revenue sharing are third-rail topics, and thus any changes to them are not concessions, thought the union did not go far enough in its proposal on Saturday. And when its negotiating team conveyed that sentiment, players were furious.

A union official, who spoke on condition of anonymity given the sensitive nature of the negotiations, said the players’ side was considering breaking off talks. But on Saturday night, the official said the sides would indeed meet again Sunday afternoon.

And so the cycle has continued. Even when both sides expressed some optimism on Friday over one matter — establishing a draft lottery for the top handful of picks in the annual amateur draft, for the first time in M.L.B. history — the union felt otherwise a day later.

In its counterproposal on Saturday, M.L.B. tied the draft lottery for the top six picks in the draft — a number closer to the players’ original request of eight — to a postseason field of 14 teams. The union, though, had agreed to expand the playoffs to 12 teams, from the current 10.

M.L.B. also incorporated the players’ concerns about service time manipulation, but tied that issue to another change it wanted — the ability to more easily make on-field rule changes, which previously required union consent or could be done unilaterally with a year’s notice. The proposed accompanying carrot for that concession: The league would round up to a full year of service time for any player who finished first or second in voting for the Rookie of the Year Award.

Despite some minor moves on the luxury tax system on Saturday — M.L.B. slightly dropped some of its more onerous tax rates, while the union slightly dropped some of its higher thresholds — the sides remained far apart.

They were also separated on the league minimum salary ($640,000 versus $775,000), a bonus pool for top players not yet eligible for salary arbitration ($20 million for 30 players versus $115 million for 150 players), revenue sharing and salary arbitration expansion.

In other words: There is a lot of ground to cover before M.L.B.’s deadline on Monday. Earlier this month, Rob Manfred, the M.L.B. commissioner, said losing regular-season games would be “disastrous” for the industry. Will Manfred, who is employed by the owners but tasked with being a steward of the game, cancel games after saying it would be bad for everyone?

While top players have been awarded record contracts in recent years, including this off-season, before the lockout, players are frustrated that their average salary of roughly $4 million has been stagnant and has not kept pace with team revenues.

They have also sought other changes, such as obtaining earlier compensation for younger (cheaper and more relied upon) players, improving competition among teams, curbing service time manipulation and injecting more spending.

This process wasn’t expected to be easy. The expired labor agreement is viewed as having further tilted the balance in the owners’ favor. Thus, when one side wants to make significant changes to the system — the players — negotiating can be laborious, tense and full of posturing.

The league, though, believes that players have a fair system without a hard salary cap and sees it as a matter of wealth distribution — that star players are disproportionately commanding more than others. While owners have listened to the union and offered ways to pay younger players more, they have also proposed some ways to generate more revenue to pay for it (such as expanding the playoffs) and some methods to curb spending elsewhere (such as their stricter luxury tax model).

(As a counterbalance to M.L.B.’s deadline threat, the players previously issued their own threat during these labor negotiations: They told the league they were reluctant to grant club owners an expanded playoffs — worth an estimated $100 million annually — if games and money were taken from them.)

The reason M.L.B. chose Monday as the deadline: It believed that a minimum spring training length of four weeks — two weeks shorter than normal — made sense to avoid a spike in injuries like the one before the pandemic-shortened, 60-game 2020 regular season. To have players and staff in place in Arizona and Florida to begin spring training by Thursday, the league said, a new accord should be agreed upon by Monday.

Among the reasons the union has bristled at that deadline: In 1990, the 32-day lockout cut spring training in half, but the full schedule of regular-season games was played, beginning a week later than usual. And last year, because of the pandemic, M.L.B. proposed full pay for a shortened 154-game season that began a month late and included doubleheaders, which players rejected.

Sensing increased urgency, the negotiating teams for M.L.B. and the union — plus three club owners and a dozen players — gathered in Jupiter, Fla., all last week, starting last Monday. The talks have produced a lot of discussion, incremental progress and many questions about what comes next.

Based on base salaries, which totaled just over $3.8 billion last season, players would combine to lose $20.5 million for each day wiped off the 186-day regular-season schedule, according to calculations by The Associated Press.

How much the owners would lose, though, is murky. But they are billionaires, so a loss of income is less of an issue. In a rare glance at the financial numbers of a successful team, the World Series-winning Atlanta Braves — the only M.L.B. club owned by a publicly traded company — reported a $104 million profit in 2021.

Monday is expected to provide some answers.