It is an international occasion, a national symbol and an individual trial for every Barca cule, every committed Madridista. El Clasico is shrouded in the fog of myth, sustained by the power of legend and distorted by the adoption of misconceptions about the two participants.
It needs no manufactured embellishment but it has assumed the title of Franco's Work Team v Catalan Rebels since the end of the Spanish Civil War in 1939.
El Clasico has become a battlefield strewn with the shrapnel of imposed agendas. It has been forensically investigated by Sid Lowe, an expert on both Spanish football and the civil war, who has stripped the Real Madrid-Barcelona match of its gaudy, lazy inaccuracies and revealed it in its full, substantial glory. His book is a work of profound research and stimulating discoveries.
Lowe has rebuilt the El Clasico as one of the great setpieces of world football on a foundation of truth. Slowly, brick by brick, reality is revealed. It is a stunning story
"Most Madrid fans are tired of the storyline of Barcelona as victims or martyrs," he says. "The full history of the clubs is much more layered than just simply Franco v Catalan nationalism."
He exhumes the forgotten president in the history of Spanish football. The football world is familiar with the story of Josep Sunyol i Garriga, the Barca president, who was shot dead by fascist troops by a roadside in 1936. But Sanchez Guerra, elected president of Real in 1935, was a victim of the trials following General Franco's victory in 1939. He was sentenced to 30 years' imprisonment. He was released in 1944, but immediately re-arrested, fleeing to exile in France.
The assumption that referees and Franco favoured Real immediately and passionately does not survive scrutiny. Real won La Liga in 1933 but did not take the title again until 1953, a full 14 years after Franco's installation as Caudillo.
"To present Madrid as Franco's team and Barcelona as victims on the basis of the civil war is to stretch the boundaries of time and to project on to the period between 1936 and 1939 events which actually occurred later, in different years and in different circumstances," says Lowe.
There can be no doubt that Barcelona provided a rallying point for Catalan nationalism as the Franco years stretched into an era but Real had to wait until 1953 before starting a journey that was marked by five consecutive European Cups and the ascension to the glory of being the most glamorous club in the world.
"In blunt terms, the reality was that Madrid were not that good," he says. "The history of the club can be summed up as before Di Stefano and after Di Stefano."
Real Madrid won two domestic titles before the Argentinian arrived and have taken 30 since. They have also won nine European Cups.
"That was the beginning of the legend," says Lowe of Alfredo Di Stefano's signing in 1953. "The history starts with him. There is a permanence attached to the greatness of Real but it did not exist until a team formed around the talents of Di Stefano."
Similarly, Barcelona's history can be summarised as before Cruyff and after Cruyff. Barca, of course, were successful in Spain, particularly after Laszlo Kubala, the wonderfully gifted and bullishly strong Hungarian, signed for the club. However, Johan Cruyff's arrival as a player in 1973 was followed by his return as a revolutionary coach in 1988.
"He was the ideological founder of the new Barca," says Lowe of the great Dutchman. He points out that Cruyff only won one La Liga as a player, but his influence as a coach was powerful, with Pep Guardiola gaining a place under Cruyff as a player and subsequently being heavily influenced by his mentor.
Yet Barcelona only won the club's first European Cup in 1992, when they beat Sampdoria at Wembley under Cruyff.
"Barcelona came late to the party in Eurpoean terms. There is a nice line from Henrik Larsson after Barca won their second European Cup in 2006. He said: 'You have to win this now and again to be a big club'."
Barca have now won the trophy on four occasions but this surely is an underachievement for a side regularly lauded as the greatest of all time?
"Charly Rexach [once Cruyff's assistant] said: 'We did not win the EC because we had this sort of veto and we were hardly ever in it," says Lowe. This speaks to the feeling that Barca were unjustly penalised by both the regime and domestic referees. It still took them five attempts to win the cup. An outstanding team can win it at the first attempt.
Lowe says: "I interviewed some of the older Barca players from the lean years and the periods when they were coming up short in Europe. I asked them what they would call a book written about the club in that period. The reply was: 'When We Were S***. There is more than an element of truth in that statement."
There is, too, a misperception about Barca-Real being a long-standing duopoly in the manner of Celtic-Rangers. Atletico Madrid and Athletic Bilbao had sustained periods of dominance before Kubala and Di Stefano brought Barcelona and Real Madrid to the fore.
"Socially they were always big teams, but their dominance only started in the second half of the last century," he says.
But they have now taken a stranglehold on the Spanish game. Since 1985 only Valencia (twice) Atletico Madrid and Deportiva La Coruna have won La Liga, apart from the Big Two.
Every El Clasico has an individual story and Lowe states the narrative this time is clear.
"Both clubs have a new manager," he says of the arrival of Tata Martino at Barca and Carlos Ancellotti at Real. "It feels like virgin territory. The departure of Jose Mourinho has been a weight off everyone's shoulders and it may be a bit calmer. But it could be the debut in the biggest game for the two great pretenders in Gareth Bale and Neymar. This match could be defined as the search for a new era."
Lowe has done El Clasico a service by defining its history.
Fear and Loathing in La Liga: Barcelona vs Real Madrid (Yellow Jersey, £18.99)