The show of unity comes at a trying time for Spain after its wealthy Catalan region attempted to break away, and where 47 percent of voters want to live in an independent, monarchy-free republic.
Credited for leading Spain to democracy after decades of dictatorship, Juan Carlos will appear at a military ceremony Saturday in Madrid alongside Felipe, who replaced him in June 2014 after a tearful abdication following several royal scandals.
"Father and son haven't been seen together at an institutional event for seven months," says Ana Romero, author of a book on the latter years of Juan Carlos's reign.
Left on sidelines
Widely lauded for his negotiating skills, political acumen and role in defusing an attempted coup in 1981, Juan Carlos fell from grace during Spain's economic crisis as his lavish lifestyle drew anger.
Adding fuel to fire, his youngest daughter Princess Cristina and her husband Iñaki Urdangarin were embroiled in a corruption and tax fraud investigation, with the latter since found guilty of siphoning off millions to fund a lavish lifestyle. Cristina was acquitted.
When he took the throne, Felipe VI, 49, took his distances in a bid to restore the image of the monarchy.
The two Kings together on the balcony of the Royal Palace on the day of King Felipe's coronation in June 2014. Photo: AFP
In June, Juan Carlos was markedly absent from an event marking 40 years since the first democratic elections after Francisco Franco's dictatorship.
Jose Apezarena, Felipe VI's biographer, says the explanation given at the time was that he didn't show up "so as not to outshine his son."
But "people close to him conveyed his unease, he was angry at having been excluded."
But that "really hurt" the man who reigned 38 years because "it was thanks to him that there was an exemplary transition from a dictatorship to a full democracy," adds Jaime Penafiel, a royal affairs journalist.
So "he made it known that if they humiliated him again, he would go celebrate his 80th abroad."
So it is that on Saturday, Juan Carlos and his wife Sofia will accompany Felipe VI and Queen Letizia to a ceremony held annually to mark the start of the year for the military.
Four years ago, that same ceremony had been a torture for Juan Carlos, physically weakened after health issues and tainted by scandals.
"He made mistakes in his speech, lost track of what he was saying, offering a pathetic spectacle," says Apezarena.
That's when he asked for preparations to be made for his abdication, he adds.
Later that year in June, he stepped down in favour of his son and became "king emeritus," a title he hates according to the press.
For Penafiel, "it wasn't what he wanted as he had repeatedly said he would be king until he died."
He would have preferred the fate of King Harald V of Norway, who is still in power and in May celebrated his 80th birthday with a big bash attended by other European royals.
So why isn't Spain doing that for Juan Carlos?
"He isn't king anymore, and the Spanish monarchy has more of a tradition of austerity than the northerners," says Apezarena.
King Juan Carlos I of Spain and Queen Sophia attend the funeral ceremony for the late King Michael I of Romania in December. Photo: AFP
Celebrations are planned to mark his birthday and that of Sofia this year, but "lower key," he adds.
At his family birthday meal on Friday, Princess Cristina and Urdangarin are expected to be conspicuously absent.
"The more they are kept away, the better it is" for the royals, says Apezarena.
And while Juan Carlos was once criticised for his lavish lifestyle -- going on a luxury elephant hunt in Botswana at the height of the economic crisis, for instance -- he is still widely admired for his role in Spain's recent democratic history.
With the passing of time, the risk of him outshining his son is dwindling, says Apezarena -- even if Juan Carlos was named world sailing champion in September aged 79.