irgendwann
someday
At an unspecified point in the future.
Irgendwann möchte ich nach Japan reisen.
Someday I would like to travel to Japan.

sometime / someday
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someday
At an unspecified point in the future.
Irgendwann möchte ich nach Japan reisen.
Someday I would like to travel to Japan.

The word is made up of 'irgend-' and 'wann' (when). The prefix 'irgend-' expresses indefiniteness, similar to 'irgendwo' (somewhere) or 'irgendwer' (someone). So 'irgendwann' literally means 'at some point in time'.
Ich weiß nicht genau, wann ich Zeit habe, aber irgendwann melde ich mich.
I don't know exactly when I'll have time, but I'll get in touch sometime.
You can tell whether the past or the future is meant from the tense of the verb. The perfect or simple past points to the past, while the present or future points to the future.
Wir haben uns irgendwann in den 90ern kennengelernt.
We met sometime in the 90s.
As an adverb, 'irgendwann' can stand flexibly in the sentence. It can stand in position 1, in which case the verb follows in position 2. But it can also stand after the verb in the middle of the sentence.
Irgendwann versteht man die Regeln.
At some point, you understand the rules.
In colloquial speech, 'irgendwann' is often combined with 'mal': 'irgendwann mal'. This makes the statement even more indefinite and casual.
Wir sollten uns irgendwann mal wieder auf einen Kaffee treffen.
We should meet up for coffee again sometime.
day
'eines Tages' and 'irgendwann' can both mean an unspecified point in the future. So you do not know exactly when something will happen. 'irgendwann' is very common and neutral in everyday language. 'eines Tages' sounds a bit more narrative and works well when talking about hopes, plans, or a later development. In many sentences, the two can be understood in a similar way.

Irgendwann besuchen wir unsere Freunde in Hamburg.
Someday we will visit our friends in Hamburg.
Eines Tages möchte Lena ein kleines Haus am Meer haben.
One day, Lena would like to have a small house by the sea.