Fluctuating Calories

I'm a 16 year old, 5'3", 105 pounds.  I walk a lot and do endurance training about 3 times a week.  I usually eat between1500 and 2000 calories a day but occasionally (maybe once a week) I eat about 2500.  I'm just wondering if it's bad for me to eat this many calories and whether it will lead to weight gain.  Thanks!

You definitely won't gain weight on that. In fact, the 2500 calorie amount sounds more reasonable than the 1500 limit based on your age and activity level.

1500 is the BARE minimum for a teen who is sedentary and trying to lose weight.

You are moderately active, a teen, and should be either maintaining or gaining (not losing). You have no reason to be eating as low as 1500 calories a day.

yummy-kitty is absolutely correct...I checked several places trying to check your supplied information to recommended calorie intake,  and I would guestion that 1500 calories are enough.   I would think 1800 would be the minimum and maybe even 2000. 

Actually 1 day of overeating helps the body to readjust BMR upwards, support muscle repair and building, restore micronutrient requirements, reset leptin and other hormones. 2 days when somone has a lower body fat %. It's actually quite important when someone is in a cronic deficit, which describes most dieters.

You are not eating enough most days. You are still growing and need more than 1500 calories a day. Fluctuating calories is OK if you are not dropping to low on the days you eat less. It is also about the kind of foods you are eating. If it is all junk at 2500 calories you will not be healthy or loose body fat. Fat and weight are two different things. And you do not need to loose any weight at all. Look into clean/balanced eating and up your calories. Make healthy choices and you will be fine.

At 16, I was the same height and weight, slightly more active (3 hr swim practices 5 days a week), and maintained on something that now seems obscene, like 3000 calories/day.  That said, everyone seems to be looking at the lower number:  OP said she eats BETWEEN 1500 and 2000 calories/day.  If that's, say, 1800, that's not so bad.  And at your age, 2500 one day/wk sounds both delightful and entirely unlikely to cause weight gain :)