One of those things/one of those days

just one of those things means something bad that happens unexpectedly:-
--I couldn't keep the dog from running toward the car. It was just one of those things.
Generally a random occurrence that can't be explained. For example:-
It wasn't their fault that the show failed; it was just one of those things.
This expression was given greater currency as the title and refrain of one of Cole Porter's most popular songs (“Just One of Those Things,” 1935). [c. 1930].
One of those days means a day when everything goes wrong, as in:--- The car wouldn't start, I lost my glasses--it was one of
those days.

one of those days: κίνησε ο Εβραίος να πάει στο παζάρι κι ήταν η μέρα Σάββατο.

one of those things: συμβαίνουν αυτά. Like these things happen or shit happens.

one of those days: όταν όλα μου πηγαίνουν στραβά, όταν έχω σηκωθεί με το ζερβό (=left foot) από το κρεβάτι

azimuthios said:

one of those days: κίνησε ο Εβραίος να πάει στο παζάρι κι ήταν η μέρα Σάββατο.
[...]

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Also «Eίπε ο γύφτος να χορέψει και σπάσαν τα νταούλια» for something bad that happens unexpectedly.

When we undertake something big (like a wedding) and, one after another, things go wrong (according to the first corollary to Sod's Law, "Anything that is to go wrong will do so at the worst possible moment"), we could also use «Όλα του γάμου δύσκολα (κι η νύφη γκαστρωμένη)». An update on that is mentioned on slang.gr, but I'm not at all certain whether it's widely used or it's just a jocular paraphrasing.

Another common phrase that comes to mind (after Dr7x's idea above) is: «Όλα στραβά κι ανάποδα».

To join the last two phrases in unwholesome matrimony :

Κουτσά, στραβά κι ανάποδα - Τάκης Μπίνης

daeman said:

[...]
Another common phrase that comes to mind (after Dr7x's idea above) is: «Όλα στραβά κι ανάποδα». [...]

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Which by the way was the Greek rendering for the title 'Clockwise' of this comedy where Brian Stimpson (John Cleese) indeed had one of those days :

An uncompromising British school headmaster finds himself beset by one thing going wrong after another.

Μια από τις σπάνιες ευκαιρίες να γιουτουμπανεβάσω κάτι...

Eagles - One of These Nights