I do not know Greek, so hopefully I am not just embarrassing myself with this question; but here goes.
For the first six seals in Revelation, John introduces them using ὅτε:
1 - Καὶ εἶδον ὅτε ἤνοιξεν... (6:1)
2 - Καὶ ὅτε ἤνοιξεν τὴν σφραγῖδα τὴν δευτέρανv... (6:3)
3 - Καὶ ὅτε ἤνοιξε τὴν σφραγῖδα τὴν τρίτην... (6:5)
4 - Καὶ ὅτε ἤνοιξεν τὴν σφραγῖδα τὴν τετάρτην... (6:7)
5 - Καὶ ὅτε ἤνοιξεν τὴν πέμπτην σφραγῖδα... (6:9)
6 - Καὶ εἶδον ὅτε ἤνοιξεν τὴν σφραγῖδα τὴν ἕκτην... (6:12)
But then on the seventh seal he switches to using ὅταν:
7 - Καὶ ὅταν ἤνοιξεν τὴν σφραγῖδα τὴν ἑβδόμην... (8:1)
(It seems that some manuscripts have ὅτε here in 8:1 as well, but given that ὅταν is the harder reading, this seems likely the result of a copyist following the pattern of the first six seals. UBS and SBL agree.)
My understanding is that ὅτε is typically translated "when" while ὅταν is typically translated "whenever." Yet it's hard to see how this could be rendered "And whenever he opened the seventh seal..." since it would suggest a repeated act. None of the English translations I checked reflect any difference here in wording from the other six seals.
It wouldn't surprise me, being that it is the seventh seal, that John might use a different temporal form, but what purpose does he have in doing so? Should the the seventh seal be understood as a repeated action? Do the English translations miss reflecting a literary feature of the text?