For 364 days of the year, the Vineyard and Nantucket sit on their respective perches in the ocean and gaze at each other with casual disregard.

The two Islands may share ocean currents, topography and even a boat line, but beyond that they have little to do with each other.

But for one glorious day of the year, these two estranged siblings shake off their shared disdain and come together for a sporting contest that is part family reunion and part grudge match.

Tomorrow is the Island Cup game between the Vineyarders and Whalers, a high school football game steeped in tradition and pride — and of course bragging rights.

“This game is like a war; it’s an entire season in four quarters,” Vineyard coach Donald Herman said. “When these kids take the field they are playing for all the players and coaches ever involved with the game.”

Coach Herman has played an important role in the Island Cup, helping to shift the balance of power over to the side of the Vineyard since his arrival from Savannah, Ga. in 1988. Prior to his arrival, Nantucket had traditionally dominated the rivalry, at one time winning seven in a row and ten out of twelve under the guidance of famed coach Vito Capizzo.

Coach Herman quickly turned that around, winning three out of the first five Island cups he coached in and more recently winning seven out of the last eight cups — including four straight, a first for the Vineyard. Just as the Red Sox now hold sway over the Yankees, the Vineyarders are now the bully in this storied rivalry.

Coach Herman said the competition between the two teams is as strong as ever, although he admits that he misses the heady days of the 1990s when the two teams played in the same league and the Island Cup game would often determine whether the Vineyarders or the Whalers made the playoffs.

But in a season like this, when both the Vineyarders (8-2) and the Whalers (3-7) had disappointing seasons and missed the playoffs, the Island Cup provides an opportunity to salvage the season and beat their biggest rival, he said.

“This game is our Super Bowl. Everything is on the line,” the coach said.

This year marks the 30th anniversary of the fabled Island Cup match, a contest that in hindsight had surprisingly meager beginnings.

John Bacheller, head coach of the Vineyarders from 1977 to 1979, said he came up with the idea for a trophy that would be awarded to the winner of the game and then stay with the team all year — much the way the Stanley Cup does in professional hockey.

Mr. Bacheller said he discussed the idea with Nantucket coach Vito Capizzo who initially gave it a lukewarm thumbs up.

“I told Vito I was going to a trophy shop in Falmouth to pick out [the Island Cup], and I told him he should go look at it,” Mr. Bacheller said this week. “He never did, so I went and picked it out anyway and brought it to the Vineyard. I think I paid something like $127 dollars for it or something . . . and when I saw Vito later he gave me $50 or something to pay for half.”

Mr. Bacheller said the idea of the Island Cup didn’t really catch on at first, and certainly had little effect on the Vineyard as they lost seven straight to the Whalers. When the Vineyarders finally won the trophy at home in 1985 under new coach Bob Tankard, Coach Capizzo had forgotten to bring the trophy with him, Mr. Bacheller said.

“He said he left the cup on Nantucket but would get it to me as soon as he could . . . he sent it over on a plane the next day,” he said.

Since then the cup has taken on an almost mythical quality, and has been the focus of pep rallies, countless news articles and essays by national sportswriters; as well as signs declaring Keep the Cup! (one of which is included in this week’s edition of the Gazette).

Mr. Bacheller said he never imagined the cup would be the focus of so much attention when he picked it out 30 years ago. “I thought it would be fun to have a trophy. I never thought it would get this big . . . if I did, I might have picked out something more expensive,” he said.

If oddsmakers were to pick a favorite in tomorrow’s contest they would have to go with the Vineyard, as the team is coming off a season in which they missed making the playoffs by a single game and were ranked number one in their division.

On the other side of the field, the once-stellar Whalers have clearly fallen on hard times, as the team arrives on the Vineyard tomorrow with a bare bones roster and even leaner record. The junior varsity game traditionally held in the morning before the Island Cup game was cancelled because Nantucket did not have enough players to field a team.

Instead, many of the junior varsity players will be forced to play in the varsity game.

Dick Herman, a former coach for the Whalers who now broadcasts games for channel 17 on Nantucket, said the team has had a hard time in recent years attracting quality players. Many student athletes now choose to play soccer, while others play in a lacrosse travel team that plays in the fall.

“It used to be easier to get the kids when we were the only game in town, now they have more choices. This year alone we had four kids [potential football players] go off-Island to play lacrosse. It takes some little steam out of the season,” he said.

But Mr. Herman said nobody should count out the Whalers, who are capable of beating anyone on any given Saturday under the direction of Coach Capizzo.

“Vito hates to lose, and he hates to lose to the Vineyard more than anything. He will do everything to get those kids up for that game. Anything can happen in this game, we’ve seen that time and time again,” Mr. Herman said.

He is right. Take the so-called water boy game on Nantucket two years ago, when E.J. Sylvia nailed a 29-yard field goal to give the Vineyarders an unlikely 21-20 come-from-behind win. But that game is more memorable because of the celebrating players, fans and a wayward water boy who had to scramble off the field during the Whalers’ last-second kickoff return.

Then of course there was the game in 1992 when the Vineyard won 14-12 after scoring two touchdowns in the last five minutes for their first win on Nantucket in 20 years.

Mr. Herman had an especially challenging task during that game because his son Peter was a cocaptain on the Whalers.

“[My son] recovered from the loss after a few days. It took me a while longer,” he confided.

But with all the heartbreak and heated rivalry over the years, many Islanders say that it’s about far more than victories and vindication — they just really like the ride.

“I always like going over [to Nantucket],” said John Stabile, a member of the Vineyard’s Touchdown Club boosters club. “I go over there with my wife and we get breakfast and everyone is really nice.”

But he adds: “Things do change a little when the game starts . . . we’ve learned to leave quickly when [the Vineyard] wins.”

One man who has an interesting perspective on the game is Cape and Islands Rep. Eric T. Turkington, who is forced into a dangerous political middle ground when it comes to the game. Mr. Turkington said he used to go to the games and stay on the home sideline for one half and switch to the visitors, sideline for the second half.

Pressed to reveal where his allegiance lies, he ducks deftly beneath his snug politician’s cap.

“I can’t go there. That’s like the third rail of Island politics . . . you just don’t touch it,” he said.

The Vineyard hosts the 2007 Island Cup game tomorrow at the Martha’s Vineyard Regional High School athletic field. Kickoff time is 1 p.m.