Is it ok to have half a multigrain bagel once a week while on a diet?

I am on a diet - but every once in a while I get a serious craving for a bagel. I satisfied this craving today by eating half a multigrain bagel for breakfast (the rest of the week I've been eating a piece of fruit and an egg).
Is it ok to treat myself with this once a week - or do you think it will upset my diet? I plan on eating healthy for the rest of the day - veggies, etc. Thanks!