I'm always fine by the early afternoon of the day after drinking the oil. ie. if you flush Friday, rest on Saturday, then enjoy your birthday on Sunday! But be kind to your body and especially your liver (ie. try not to drink alcohol, eat meat, etc.).
I know what you mean about the cold, hard, facts. There IS lots of misinformation on the Internet regarding liver/gall bladder flushing. BUT, my own experience and hundreds of other posters proves that Liver Flushing does transform your health.
Like I said, I've flushed several times over the last ten years. I've never released any hard stones but I've released lots of solidified bile that was chogging my liver and my health has been transformed (chronic fatigue and Acne cleared up, allergies cleared up, clean tongue, soft pink supple skin, more energy, better sleep and so on).
On the other hand, I'm sure it won't hurt you to wait a week either! Follow your instinct and Happy Birthday!