Short, but there you go. The Scottish phrase auld lang syne roughly translates to "old times", "days gone by", "times long past", or whatever your linguistics professor feels like.
Professor McGonagall sat before a fireplace, a tartan shawl loosely draped over thin shoulders. An untouched glass of firewhiskey sat on a nearby table. She had poured it in celebration of Voldemort's defeat, but the first sip was bitter, reminding her only of lives lost in the war, not least of which were Lily and James'. She would not drink to the deaths of her students—not in celebration, nor in grief.
Instead she read from a book of poetry, a worn volume of Robert Burns, the celebrated Scottish muggleborn. It sat in her lap, as it had for some time, open at Auld Lang Syne:
Should auld acquaintence be forgot,
And never brought to min'?
Should auld acquaintance be forgot,
And days o' lang syne?
For auld lang syne, my dear,
For auld lang syne,
We'll tak a cup o' kindness yet,
For auld lang syne.
The professor laid the book aside, next to her drink, and rose to her feet. She would check on Harry Potter one last time, despite the Headmaster's assurances. One last time, for old acquaintances and times long past.
She arrived on Privet Drive with a soft crack, knowing (as she always did) that her feet were no longer on Scottish soil.
The evening was early, but dark. No one could have hoped to see the professor turn into her feline form and pad towards the Dursley residence. She leapt upon the white picket fence surrounding Petunia's well-manicured lawn, turning her cat ears, magically sensitive and searching, toward the house.
A baby was crying. McGonagall's tail twitched.
"Pet!" roared a slurred, male voice, "I told you to keep that...that freak quiet!"
A woman replied, though not in answer. She was angry, afraid, frustrated. "Shut up! Shut up!"
Something was slapped. Flesh on flesh, sharp as a gunshot to McGonagall's ears. For a second, a split-second, the baby's crying stopped, only to burst out more desperate than before.
The professor jumped down from the fence, feline grace contrasted by the sloppy, sharp shift from her animagus form. Words were being said inside the house, but Minerva McGonagall didn't hear them. She only heard her heartbeat, strong and fast. A bucket of ice water had tipped over in her stomach, spreading to her feet and clenched, trembling fingers.
Whether she began to walk, or the earth began to move beneath her feet, the transfiguration mistress couldn't say. Forward though, forward she went, smooth and unrushed. Before her grass turned to shamrocks, while behind became a path of needles.
Paving stones rippled as she she mounted the porch. A front door turned to tissue paper posed no obstacle as she pushed through, entering the house.
Cries came from the left. She made straight for the plaintive sound, not pausing at the wall standing in her way, not blinking as it folded away with snaps and creaks of groaning wood.
She found herself in a kitchen. Adult shouts layered over infant sobs. Petunia Dursley stood at a table, next to a familiar bassinet, screaming at McGonagall's entrance. Vernon Dursley struggled in the attached living room, trying to get up from a chair, bellowing in righteous anger.
A waved wand, drawn without a thought, and the Dursley's went unnaturally still, controverted expressions of horror and anger frozen in place.
In the sudden silence, McGonagall approached the bassinet. There lay Harry Potter, just as she remembered him, save for an angry red welt on his cheek. She took him in her arms, cradling him, and began to softly sing:
"Because these green hills are not Highland hills
Or the Island's hills, they're not my land's hills,
As fair as these green foreign hills may be
They are not the hills of home."
With only a suppressed crack to mark their exit, the two had gone. Away from a beautiful house owned by the ugliest of people. Away from the very shores of England. They appeared far west, in the deepest Highland hills, next to the Cross of McGongael. The weathered marker stood at the edge of ancient Gaelic wards, faithfully watching over the borderlands of Clan McGonagall, a land where the old laws of gyrth and sanctuary still held.