Happy Birthday to her!
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For tonight, you have two choices:
1) Don't start your diet tonight - this may not be the right time because of the party. Go ahead and eat. Start diet tomorrow.
2) Eat a good dinner but simply refrain from the cake, ice cream, etc.....every time you are tempted, create a mental picture of yourself when your weight goal is reached and recommit to getting there. This type of discipline gets easier the more you practice it, especially when the pounds start coming off and it starts to become reality.
Either way, I'm pretty sure you'll do the right thing for yourself because your presence here on 3FC tells me that you want to try. Good luck!