It's an excellent idea to assign a day for quitting smoking and when the time is ripe and you are sure that you are going to quit smoking, you have overcome the biggest barrier, as trying to quit smoking without real desire, is not possible at all. The quit smoking process is not really as difficult as you think; once you've made your mind up to quit smoking, it's just like going on a diet. A little more challenging, as smoking is an addiction and like any other addiction, is very tough to break.
During the first 72 hours of quitting smoking process you will need to drink plenty of water or acidic fruit juice to help move out the alkaloid nicotine buildup in your body and to alter blood sugar levels. One of the best juices is cranberry juice, which will do the job and supply your body with vitamins and nutrients.
Fresh fruit and vegetables are good choice too, along with whole grains, seeds and nuts. You should be able to avoid gaining too much weight if you eat reasonably; weight gain is often related to quit smoking process, this is because not only are you tending to eat more to fight cravings, but also because of increased by smoking metabolic rate you will burn calories quicker, so apparently when you first quit you should try to modify your calorie intake, at least until the metabolism goes back to normal.
The best way to quit smoking is to take it one day at a time; follow an exercise routine, even if it's only walking, and find something to keep you occupied. Keeping a diary or blog is a good idea, as not only does it give your hands something to do, but you can keep an account of your progress; how you felt, how hard the day had been, things you had to drink or eat and how they helped, and so on.
Get something to snack or nibble, supply yourself with plenty of healthy snacks: grapes and cherries are good, or anything small which you can pick at separately, or sticks of celery are ideal, as not only will celery help calm cravings, but the body really burns calories while digesting it! So there's a double bonus in eating celery.
The decision to quit smoking was a challenging, but having done so proves that you have strength of character and willpower, so you should be able to stick to your plan without too much trouble. Trial and error will find the best way to quit smoking for you, but if you find out you need help, then that's OK too. There is always a solution and a real way to quit smoking.