Smoot hit the nail on the head.
The only way to actually know when an EM is going to drop an item is to observe the EM's event style. You have to remember that EM's are a paid creative position. As with anything creative, there is a style to the masterpiece.
Lets look at this from a different perspective shall we. Lets introduce the technique of Leonardo Da Vinci:
Leonardo da Vinci typically painted with oil paint that he made by hand from ground pigments; later in his career, he worked with tempera made from egg whites. His work surface typically would be a canvas or board, or sometimes stone when painting a mural. As da Vinci began a painting, he would start by covering the canvas with a pale gray or brown, using the neutral color for underpainting. Atop of the underpainting, da Vinci would layer transparent glazes within a small range of tones. Typically, the colors used were natural hues; da Vinci never used intense or bold colors or tints in contrasting colors. By using such a small range of colors, he was able to give his finished works a more cohesive appearance.
How does this coincide with Event Moderator Technique?
Leonardo's work could be noticeable either by a distinct signature or by his very known technique. The same principle applies with Event Moderators. Once you figure out the EM's event technique, you can pretty much land when the event drop will be.
EM's Signature: "Main Event"
EM's technique: (look below)
Lets use Great Lakes as an example. These EM's are worlds apart from the other EM's so they catch on and change their technique occasionally to remain fresh and unpredictable. However, a common technique that rarely changes that I have noticed is an event at the first part of the month and an event at the later part of the month. Now since Great Lakes changes their technique some, the day of the drop is mostly guess work. To determine such dates, you look at the name of the event and then look at past event story fiction to see if the event name gives off a principle of importance. If the importance aligns with the current story fiction, then there is a good probability of a drop.
DISCLAIMER: I've been attending EM events for over 6 years. I got to every event, and not just the ones I 'think' will have a drop. I go because I love the creative style of the EM's and the fiction they provide. Now, before I get labeled a filthy rares collector, let me acknowledge that Rares Collecting is indeed a playstyle like PVP and PVM is. I attend events, get an item here and now and then, and if it doesn't fit my collection, I sell it to have funds to purchase other items that fit my collection. It's a play style and a business, and the first step to learning how play a playstyle is to learn the business of such.
