The turnip calculator predicts the best selling price during the week. The prediction is based on the daily price information provided to the skill. The prediction is not a guarantee, it's only an approximation.
The prediction algorithm was taken from: https://gitlab.com/nanoNago/turnips which in turn is based on: https://gist.github.com/Treeki/85be14d297c80c8b3c0a76375743325b