Couples will convert existing civil partnerships or set the seal on their relationships for the first time in Berlin, while others exchange rings in Hanover, Hamburg and other cities.
Local authorities rushed to get weddings underway as soon as possible, after lawmakers voted on June 30th to give Germany's roughly 94,000 same-sex couples the right to marry.
But German bureaucracy being what it is, government software will be unable to officially record two men or two women as married until next year - meaning some online paperwork will still register them as "husband" and "wife".
"Finally our country is joining the rest of Europe!" said Jörg Steinert, head of gay and lesbian rights organisation LSVD in Berlin and Brandenburg state.
The Netherlands was the first country to legalise gay marriage in 2000, followed piecemeal by 14 European neighbours like Spain, Sweden, Britain and France.
But Germany made do with a 2001 civil partnership law, extended over the years to remove more and more gaps between gay and straight couples' rights.
That was "a first breach in the institution," Steinert said, paving the way for Sunday's "very symbolic step."
"We won't be a second-class couple any longer," Bode Mende, who with partner Karl Kreil will form the first couple to marry in Berlin, told newspaper Neues Deutschland Thursday.
Mende and Kreil, together since 1979, have for years campaigned for equal marriage rights.
The law now reads "marriage binds two people of different sexes or the same sex for life".
By extending existing law to same-sex pairs, they automatically gain the same tax advantages and adoption rights as heterosexual families, avoiding the endless back-and-forth in some nations over adoption.
Along with the Greens party, the LSVD began its battle for equal marriage rights around the year 1990.
By 2017, same-sex relationships have become so normalised that polls show around 75 percent of Germans are in favour of gay marriage.
Unlike in France, there were no rallies of hundreds of thousands against the law.
"Lots of people were amazed by the end that it hadn't already happened, asking themselves, 'surely we have that already?'" said MP Johannes Kahrs, gay and lesbian affairs commissioner for the SPD - who himself will act as witness in a close friend's wedding Sunday.
'Thanks for nothing!'
Lawmaker Kahrs enjoyed a flash of fame in June, when he laid into the snap decision by Chancellor Angela Merkel allowing conservative MPs to follow their conscience on a gay marriage vote - the trigger for the rush to pass a bill.
"Thank you for nothing, Frau Merkel!" he stormed, pounding the lectern in the Bundestag (lower house of parliament) with rage.
Merkel explained her thinking changed after a "memorable experience" when she met a lesbian couple who lovingly care for eight foster children in her Baltic coast constituency.
Her surprise shift in position - after 12 years of blockade by her Christian Democrats and their Bavarian allies - was seen by some as a cynical ploy to rob her challengers of a popular cause ahead of September's election.
The chancellor herself voted against the bill, arguing that the German constitution still defines marriage as "the union of a man and a woman".
"I still think it was indecent to delay for so many years, and the fact that she voted no," Kahrs told AFP.
Even now, the conservative Bavarian government has put experts to work investigating a constitutional challenge against the law.
But Kahrs is confident that a case will never be brought - or, if it were, that judges would uphold gay marriage.
Long way to go
June was a bumper month for gay rights in Germany, as MPs also voted to quash the convictions of thousands of men convicted under a Nazi-era law against same-sex relationships which had remained on the statute book until 1994.
But there are still an array of issues familiar across western democracies, like blood donation or access to reproductive medicine, where homosexuals can be treated differently.
And the constitution - which forbids discrimination based on sex, parentage, race, language, homeland and origin, faith, religious or political opinions or disability - must be extended to protect against discrimination over gender or sexual orientation, Kahrs insisted.
"These are all things that we'll tackle bit by bit," the MP said.
"The important thing is that we've pushed through the opening of marriage, and that's the signal everyone needed."