I figure the ‘day of the Lord’ refers to the Millenial Kingdom that begins with the Second Coming in Rev 19 and the ‘battle’ of Armageddon followed by the 1,000 year reign (the Sabbath of the Earth - Rev 20). That is why there is the reference to the ‘thousand years is as a day’ in those verses in 2 Peter. It appears to be a clue that the ‘day of the Lord’ is 1,000 years long. At the end of that ‘day’, the heavens pass away, the earth is burned up in Rev 20:11 and the judgement begins followed by the new heavens, the new earth and the New Jerusalem.
Otherwise, it looks like 2 Peter is saying that the Second Coming doesn’t happen until Rev 20:11 and that is too late for the martyrs to reign w/ Christ during the Millenial Reign and the 1,000 year Sabbath of the Earth.
Or, more likely, there is no millennial reign on the earth for a thousand years. At least there is none recorded in Rev. 20.
BTW, 2 Peter 3 does not say that the "day of the Lord" is a thousand years. You only get that by allegorizing the text. Something, I understand, futurists are opposed to.