We know who is making some more batches of soap in the next two weeks
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Joke aside. The general recommendation with best-by dates is that it's a good orientation how long the manufacturer expects the oils to not have become rancid. Given some assumptions about how customers would store it (dark, cool/constant temperature, dry, little stirring, away from metals). For the few pitiable soapmakers who don't happen to have quick access to an NMR spectrometer or gas chromatograph, the manufacturer's best-by date is the only positive clue for suitability of an oil (mind also liability matters), hence the rule-of-thumb that best-by date of oils = best-by date of soaps thereof. Unfortunately, without fancy apparatuses one can usually find out if the actual no-longer-best-after date of an oil has passed when it's too late (aka DOS).
Now, the manufacturer might have underestimated how tenderly you of course are treating your raw materials. In particular with rather robust ingredients like palm oil. “One year” is a bureaucratic compromise for “one year for sure, after two years we have observed a few write-offs under adverse conditions; so let's go with 1 year to be on the safe side”. No need to panic FWIW. Surely, a sniff and look on the oil can detect deterioration only when it's quite progressed, but chances still are that it's okay for soapmaking as late as December 19 or even beyond.
Some other indicators for rancidity (for food-grade ingredients) are a bitter/acrid taste, unusually low smoke point (put some in a pan/pot and heat it up on the stove)/unpleasant smell when heated, and bad glide/adhesion when baking/frying scorch-prone food like pancakes or roast potatoes.
When still not convinced, bake cakes with it, or biscuits, or make meat fondue.