Can we know what “day” Hebrews 10:25 is referring to?
Hebrews 10:25: "[Not] forsaking our own assembling together, as is the habit of some, but encouraging one another; and all the more as you see the day drawing near" (emphasis added).
Yes --It refers to the "Day of Jehovah"
At Pentecost of 33 C.E., Peter explained that what was occurring was the fulfillment of Joel’s prophecy (Joel 2:28-32 read below on link) concerning the outpouring of God’s spirit and that this too was due to happen before “the great and illustrious day of Jehovah.” (Acts 2:16-21)
That “day of Jehovah” came in 70 C.E., when, in fulfillment of his Word, Jehovah caused the armies of Rome to execute divine judgment upon the nation that had rejected the Son of God and defiantly shouted: “We have no king but Caesar.”—John 19:15; Daniel 9:24-27. This is the first fulfillment, there is also a further fulfillment.
Some miniature "Days of Jehovah" that befell Judah and Jerusalem. Read Isaiah 2:11-17, Ezek 13:5, Zep.1:4-8
Zephaniah 2:2-3 ASV
2 before the decree bring forth, [a]before the day pass as the chaff,
before the fierce anger of Jehovah come upon you, before the day of
Jehovah’s anger come upon you. 3 Seek ye Jehovah, all ye meek of the
earth, that have kept his ordinances; seek righteousness, seek
meekness: it may be ye will be hid in the day of Jehovah’s anger.
Zephaniah 1:18 ASV
18 Neither their silver nor their gold shall be able to deliver them
in the day of Jehovah’s wrath; but the whole land shall be devoured by
the fire of his jealousy: for he will make an end, yea, a terrible
end, of all them that dwell in the land.
Ezekiel 7:19 ASV
19 They shall cast their silver in the streets, and their gold shall
be as an unclean thing; their silver and their gold shall not be able
to deliver them in the day of the wrath of Jehovah: they shall not
satisfy their souls, neither fill their bowels; because it hath been
the stumbling block of their iniquity.