Story Arc: Fanatic and Apostate's First Fight
Though it is likely that Fanatic and Apostate have fought multiple times over time, this one can be placed with some degree of assurance, as it involves a fight between the two, but has the first appearance of one (Fanatic) in one book (Mystery Comics) and the first appearance of the other (Apostate) in a different book. The fact that each has their first appearance in a book with the other means that those first appearances must closely overlap.
Fanatic #25 is the first appearance of Apostate, and is also referenced on two cards, "Condemnation" and Corrupted Effigy. References also appear on Periapt of Woe for #26, and on Zealous Offense, Remorseless Provocation and Gauntlet of Perdition for #27. The appearance of Nightmist in the quote for Periapt of Woe makes it possible that Nightmist is also involved in this arc.
Mystery Comics #367-370
Mystery Comics #368 is the first appearance of Fanatic, fighting Apostate. References appear on Smite the Transgressor and V-AP-TU for issue #368, on Chastise, Fallen Angel, Orb of Delirium and Fiendish Pugilist for issue #369 and Runes of Malediction, Rocket Punch and Karmic Retribution for issue #370. These last three references also make it possible that Ra, Omnitron-X and Setback were involved in the fight.
The presence of Nightmist in the quote for Fanatic #26 makes it a possibility that these issues line up as well. Absolution #18 has Apostate in the art, and a quote from Nightmist (Astral Premonition). Absolution #19 has multiple references - Wrathful Retribution, Profane Summons, and Apocalypse. The further fact that Redeemer Fanatic came just after being "confronted by the deceiver, Apostate" (>G bio for Redeemer Fanatic), and the Fanatic in the art on H-F-WR looks like the original Fanatic lends credence to this all being one event.
This one is actually a bit of a mess. As discussed above, the first appearance of Fanatic is in Mystery Comics #368. With Apostate, who shows up on the art on Smite the Transgressor and is the quoted person on Tome of the Unknowable, both of which reference #368 (he's also quoted on Mystery Comics #367 - before his first appearance - Holy Nova). However, Apostate's first appearance is in Fanatic #25, a book that one would think would involve Fanatic from issue #1. It is possible that either 1) Fanatic #1-24 involve the telling of a story without Fanatic herself actually being involved until issue #25, which was published after Mystery Comics #368, or 2) Fanatic #1-24 involves the telling of Fanatic's substantial backstory before she was technically Fanatic. Then Fanatic herself first appears in Mystery Comics #368, as posited in hypothesis 1.
Two other side notes here. It should be noted that aligning these two comics in this way, combined with the Dreamer alignment, above, does allow for The Wraith's first appearance in Mystery Comics #27 to occur before Freedom Five (Four?) #1. Also, though Apostate does appear in Nightmist's own book (#6 - V-AP-IP and V-AP-RS and #7 - Mistform and Offensive Transmutation), that appearance will only align with this event if the telling of Nightmist's book stretches over substantial time, given that issue #51 is the conclusion of the Invasion Day arc and the alignment of her eponymous book with Freedom Five through that arc and the alignment of Freedom Five with Fanatic through the Dreamer arc. Much more likely is that Apostate's appearance in issues #6 and 7 of Nightmist are unrelated, as there is no appearance of Fanatic in either the art or quotes of those issues, and the only other hero appearing, The Scholar, is in none of the issues above.