I have added a new feature to this reading order in the formatting. Bold means main event series, underlined means tie-in written by Kieron Gillen, and italics means tie-in that I think fairly important/essential. This gives you four levels of reading the entire event: just the main series, just the Gillen stuff, just the stuff I think is good/important/essential, and the entire event including all of the tie-ins. That italics group is entirely my own taste/judgment and you may not agree.
While not completely essential, before we even get into Judgment Day, I truly would advise reading Eternals and the first three issues of Immortal X-Men. Eternals #1-12 along with the specials, in particular, inform Judgment Day. The event basically began as the third storyarc of that title after all. The Immortal X-Men issues are less essential, but they’re quite good. Mostly, you should read all of these comics before reading the event because they are good comics.
* Free Comic Book Day 2022: Avengers/X-Men #1, X-Men: Hellfire Gala #1, and Immortal X-Men #4. These all set up different elements of Judgment Day. The FCBD stuff that really ‘matters’ is mostly repeated in Eve of Judgment, while the two X-Men comics are nice background on what’s going on with the mutants right before the event starts.
* AXE: Eve of Judgment #1. The proper prologue to the event that gets listed on the official checklist.
* Judgment Day #1
* Immortal X-Men #5 and X-Men Red #5: Both issues expand upon scenes in Judgment Day #1, but Gillen wrote Immortal X-Men, so it comes first.
* Judgment Day #2
* Death to the Mutants #1 and X-Men #13: Expansions on Judgment Day #2 and Gillen gets priority. But, I genuinely think X-Men #13 works better after reading Death to the Mutants.
* Judgment Day #3
* Death to the Mutants #2 and Immortal X-Men #6: I find that Death to the Mutants #2 is a little more related to/expands upon the events of Judgment Day #3 and would suggest reading it prior to Immortal X-Men #6.
* X-Men #14, X-Force #30, Wolverine #24, X-Force #31, Wolverine #25, X-Force #32-33, Avengers #60, Amazing Spider-Man #10, Fantastic Four #47-48, AXE: Iron Fist #1, and Captain Marvel #42: For this bunch of judgment-related tie-ins, the order does not matter really, except in the case of the X-Force and Wolverine issues. You can shift that chunk of six issues wherever you like in this group, but I would recommend keeping them in the order that I have here. X-Men #14 is a borderline important issue as the final three pages are referenced later on in the event, but the rest of the issue is completely unrelated. However, I wouldn’t call the reference essential or something that isn’t explained enough later on to warrant putting that issue in italics.
* X-Men Red #6: This issue leads directly into Judgment Day #4 and is best read right before it.
* Judgment Day #4
* X-Men Red #7, Legion of X #6 and Marauders #6: X-Men Red #7 is a nice expansion on the Storm/Magneto scene from Judgment Day #4 and, then, largely unrelated to the event anymore. Despite Legion of X #6 expanding upon a moment in X-Men Red #5 (which expanded upon a moment in Judgment Day #1), it ultimately takes place after Judgment Day #4, covering a lot of ground. The Marauders issue was released fairly early in the event, but takes place around the effort to reopen the Arakko gate, so it seems best placed here.
* Judgment Day #5
* Immortal X-Men #7, Death to the Mutants #3, and AXE: Starfox #1: The ‘outside the Progenitor’ tie-ins between the final two issues. This order is based on Gillen’s advice for reading them. Who am I to disagree? (Especially when I don’t.)
* AXE: Avengers #1, AXE: X-Men #1, and AXE: Eternals #1: The ‘inside the Progenitor’ tie-ins between the final two issues. These three issues are more directly related to the resolution of the event in the final issue and seem better suited to read right before it.
* Judgment Day #6
* Judgment Day Omega #1: The epilogue to the event and Eternals.
Hopefully, this is a help to anyone looking to make sense of how to read the event and its various tie-ins.