Gilda Dent as the original Holiday killer makes perfect sense. She is a very slight, short woman, and while not unattractive by any means, is rather plain compared to the likes of Selina Kyle, Carla Viti and certainly the imposing but vivacious Sofia Gigante. She's not going to easily stand out in a crowd- she could sneak in (or out) of anywhere with little effort. She could easily sneak into Johnny Viti's home and shoot him dead, shoot Milos on Falcone's stoop in the wake of the chaos created by The Joker, and even blow away all five of "The Irish" at a Thanksgiving dinner table, after they'd probably been drinking for quite awhile and totally off their guard. And even if a target sees her coming, who's going to consider this 5 ft (at best), extremely petite woman a threat?
The same is true for Alberto Falcone, to a lesser degree. A physical wimp by any standard, and well known to be outside and uninvolved in any Mafia dealings, he could also easily approach the remaining targets (Carla Viti, the Gotham City coroner, even Maroni's men), and wipe them out, with the combination of an unassuming air, and Refuge in Audacity. But Harvey Dent, Gotham's crusading District Attorney, and the man most hated and feared by the criminal community next to Batman, simply COULD NOT walk up to any of these targets with the stealth and/or impunity the others could and did.
Why no killing on Easter? Simple: It's all about rebirth and resurrection, so it woudn't match the theme of any killing (despite being a bigger holiday than April Fools)
Easter follows a lunar pattern as well, as while most times the holiday falls somewhere in April, it can vary by several weeks. April Fools, however, is always on April 1st, and thus the more chronologically reliable.
