Indeed, this might be the worst time of year to be on a diet.
Although I have never been on a diet myself (I'm real skinny), I used to work in a health food store and one of my tasks was to help people with their diets and offer advice.
Whatever you do, do NOT go on a low carb diet, unless you are willing to stick to it for ever and ever.
Sometimes it is not about what we eat, but it is about how much we eat and when we eat it. Eat small meals several times a day rather than one or two big meals a day. Find some low calorie items you like. If possible, don't eat out. Nearly all restaurants use way too much butter and other fattening stuff. Have enough exercise. The more you move around, the more calories you burn. When you see a piece of pie, endulge, but only eat half of what you would usually eat. That way you still get to eat the pie, but you will only get half of the calories. If you take your coffee with sugar, only put half the amount of sugar in it. It will taste different for a few days but you would be amazed at how quickly you get used to the taste. After a while you won't even be able to drink coffee anymore with the amount of sugar you used to put in there. Avoid pasta, it is an incredible source of calories. You don't need chips, they have lots of calories and cholesterol. Eat carrot sticks instead, or a low calorie cookie. Don't tell yourself that there are things you will never be able to eat again because they have too many calories, telling yourself this will only produce a craving for such things. The stricter you make your diet, the more chance you have that it will fail. Don't deny yourself everything all at once, leave some room for cheating, it is not a sin.
Just always remember that you need to burn the calories you take in. When you eat late at night and then go to bed, your body will store the calories as fat cells, no matter how strict your diet was duting the day.