That is an interesting question. I can give you my opinion and what I would do but it is not necessarily based on science, just my idea.

Since you are non-diabetic I presume your pancreas is producing insulin appropriately to control blood sugar, so I would say go ahead and have your birthday treat with a nice big piece or maybe even a couple pieces that day, then go back to your routine.

My reasoning is our bodies evolutionary are designed, I believe, to take in seasonal high loads of glucose/fructose. If you think back to a time of hunter gatherers fruit would have only been available seasonally. The body is designed to handle occasional high loads of glucose/fructose. Fructose is nearly all processed into fat within the liver. That is why NAFLD (non alcoholic fatty liver disease) is so prevalent in today's population. Look at people that are otherwise fit, trim and not too much over weight but have a pot belly and likely it is their enlarged liver pushing out their belt line. Fructose, the sugar in fruit and table sugar is half fructose, is a major contributor. All that "healthy" high "fructose" sugar fruit. But our bodies are designed to handle it in either modest quantities or seasonal large amounts. It is stored in the liver as fat then converted to energy when we are not eating.

Some even argue that it is even desirable, when on a ketogenic diet, to occasionally go off and have some more carbs so the body "remembers" how to handle those larger carb loads. What I believe a person wants is not to be either a fat burner or a carb burner, but instead be metabolically flexible and be able to switch fuels quickly and easily. So there is some argument that the occasional higher carb meal is actually desirable to keep the metabolic machinery will oiled.

One thing you may find though, if you have been on a very strict low carb diet for a long time having a huge carb intake may cause you intestinal distress. I have heard reports of people being keto or carnivore then have a blowout meal of something like pizza and pay for it in feeling really bad for a while. So that is something else to keep in mind.

I am diabetic but control now with diet with pretty good glucose numbers. We stay generally under 20 grams of carbs a day but occasionally (especially if we eat out) might hit 30 once in a while. Last year at about this time after being on the diet for a half year or so I did eat a piece of pie for my birthday. Checked my sugar after and it stayed reasonable. That told me two things. I could have the occasional splurge and not spike my blood sugar like back when I was a lot more insulin resistant and it did not cause me any gastrointestinal distress. Both good.

That is a long rambling semi-answer. But for a person that is metabolically healthy, a piece of pie isn't going to kill you. Have it, enjoy it, then go back to what was working for you to meet your goals. That is just the way I look at it. May not be the right way.

John