“Friday is the best day to sell” is already marginalised in the Investopedia article dedicated to it[1] .
The assumption “if everyone is selling” cannot be true, when there are only people willing to sell there is no trade, and with that no price.
It same assumption also includes a notion that there are fewer parties that are willing to hold over the weekend than there are willing to sell. In other words: it implies there are more short term traders than long term traders and (buy and hold) investors.