How to plan events....
1) "WHEN" Communication!!! First of all, the guild should select specific days of the week as "Raid Days". This works far better than using a guild calendar and scheduling events on it for several reasons. The Calendar might not be updated early enough for everyone to see the event and plan to be at the raid. Many of the members may not have the Calendar add-on installed. They might not know they even need to get one, or they might have been told, and didn't find it to be "essential"... and still have not bothered to get it. The Calendar is not necessary if days are selected and remain static. The "Raid Day" method always seems to work better because of communication. If someone us unsure of when the raids are, almost anyone else in the guild can tell them exactly what days are the "Raid Days". If the Calendar is being used, usually only one person, or a handful of people will actually be scheduling the events and times. There may even be a miss communication problem between the people scheduling them causing confusion.. Again.. just simplify and have "Raid Days". For example, My guild will always raid on Sunday, Monday, Tuesday, and Wednesday. Raiders are expected to show up for the raids, and should let raid leaders know if they can't come.
2) "WHAT" Communication.... So the first step is knowing WHEN, now what must be communicated is knowing WHAT to expect... This is best done through a simple guild message of the day explaining what the raid will be, and what any other future details.. Also, our raid leader gives a very precise summary of what we will expect to accomplish at the beginning of the raid, and at the end of the raid, his evaluation of how well we did, and what to expect to happen in the next raid day, as well as any type of consumables or equipment that will be needed.
Even with great communication expressing raid times and expectations, there still might be problems with raiders not showing up. Some raiders will quit playing or leave the guild, or their computer breaks down, or they just have a lot going on in life that pulls them away from raiding for a period of time... These things are to be expected, and it seems recruitment is an on going process.