Etiquette doesn't have any flexibility on the rule about which social engagement takes priority. It's the one you committed to attending first. People, however, typically are more flexible in that respect.
So if you have an extraordinary circumstance that comes up where you feel you really must break a prior engagement, you don't just break it and assume everything will be okay. You acknowledge that you are, in fact, acting in a way that is contrary to good manners and apologize for doing so. You count on the fact that most people you'd want to set a casual lunch date with would understand that you felt you needed to break it to attend a wedding that was moved up for reasons that are both important and impossible to predict.
The point is not that you can't back out of invitations. The point is that a) you are backing out of one, no matter how understandable, and b) you will not always escape negative repercussions for doing so. Because it's not polite, and so it is a transgression of manners that requires a certain level of forgiveness, one which may not be extended if it is requested too often.
In addition, the way that a host treats his or her invitations is the thing that communicates to the guests how important the event is. I don't tend to plan casual lunch dates more than about a week in advance, if that. Because if it doesn't work out that week, it's no big deal to reschedule. There's no specific date or time that it needs to happen on in order to be successful. But at the same time, if it's important for me to have lunch with a specific person, I do typically have to give at least a few days notice, even though planning the lunch date rarely takes more than a 5 minute phone call.
On the other hand, birthdays, anniversaries, and other similar events do have specific dates that matter. They can't necessarily be rescheduled for a different week or a different month because of scheduling conflicts. More importantly, they involve gathering more than two people in one place at one time, which is statistically much more difficult to manage than just trying to find a time when two individuals have an hour to meet for lunch.
So for that type of social engagement, either the date and attendees are important enough that people would expect to set aside other things they'd rather do in order to be there, or they aren't. Saying that a party is important enough that people should back out of invitations they've already accepted, but not so important that anyone needed to be told the party was happening more than a few weeks in advance, is contradictory. Those two things are mutually exclusive.
I also think that the advance notification of guests is an element of party planning that is every bit as important as making sure those guests have food to eat. It isn't necessary to send formal or even particularly detailed invitations well in advance if you're planning a party. But if you want people to be able to plan for it, you do have to at least say, "Hey, my 50th anniversary is coming up this July, and we're planning to throw a party. We're still figuring out the details, but we'd like you to be able to attend, so we wanted to give you a heads up."
If you give that informal heads up 2 months in advance and then do nothing else at all until 2 weeks before the party, it still took you 2 months to plan it. It just didn't take you 2 months of doing nothing but planning.