How to prepare for 24hr milsim event

Lets start with the basics: gear.
Usually 24hr events involve a separate place or off zone for camping/place to get some rest, that means you should pack a tent, sleeping bag and highly advisable some mat to sleep on.
Next are the clothes, rather take one shirt, pants, etc. too many, because you will get wet one way or another, either from sweat or if you are unlucky rain. Besides most players on the field take a short rest before the night time to prepare the gear/change clothes so keep in mind taking a set of clothes for night time if you are planing to play.
Now for the fun part, the gear you will most likely carry for the next 24hrs.

First and most important thing you will carry all the time in airsoft is of course your gun. It is advisable to pack at least 2 batteries for your gun, and keep a spare for your optical device if needed.
Next thing, keep your gear weight according to your physical endurance. Better to keep it light, because even light gear becomes heavy after 12hrs or so. As far as the pouches go on your rig, there is only one that should be necessary and that is the first aid pouch. You never know what can happen on the field, and although there is first aid almost always available at such events, maybe that bandage might come in handy if you for example cut yourself.

One more thing that is really important is to have a camelback or a backpack in which you can put enough water in (recommended at least 3L). Backpack also comes in handy if your team decides to go more authentic and you eat on the field, in that case MREs are the way to go. But if you decide you will eat in the off zone, than don’t forget the propane burner and usually a dish to put your food in.

While you’ll be patrolling, crawling and waiting around remember to have fun, but be careful about sound discipline, someone may be listening.

Next we have the night time of the event.
For the night time you will need tracer and tracer bbs, or imitation of fire (you can use normal bbs in that case), you will also need a lamp and a red lamp for marking yourself if you get hit (really great option are LED dog collars that are sold in any pet store).
Night time is really difficult if nobody invested in NVGs or thermal visions, so you will have to use your lights which can give away your position really quickly, try turning it on while obscuring it with your hand. But whatever happens, do not run in to the dark because your ankles and knees will not be grateful.