Stock prices do not exist on a Saturday. The exchange is closed, nothing trades, and the Chainlink feeds that Pillar reads from update through the trading week and then stop. The last print on Friday evening is the price until Monday. This is a small, boring fact about equities, and it is one that a lending protocol built on them has to take a position on, because the code will run on Saturday whether or not the price does.
The position Pillar takes is this. Every market has a staleness limit, and it is set to eighty hours: a full weekend plus a margin for holidays and slow feeds. Within that window, the Friday price is the price, and everything works. Past it, the oracle reading is treated as stale and the protocol declines to act on it. You cannot open a new borrow against a stale price. You cannot withdraw collateral against one. Nobody can liquidate you against one either, which is the half of this rule that protects you.
What still works with a stale price is everything that does not depend on it. Deposits work, because adding collateral can only make a position safer. Repayment works, because paying down debt can only make a position safer. Harvesting yield works, because it does not touch the price at all. The rule is that the protocol will do anything that moves risk down and nothing that moves risk up while it cannot see the market.
There is a case people worry about, and it deserves a direct answer. Suppose a stock gaps down over the weekend. The Friday price says your collateral is fine; the Monday open will say it is not. Between the two, Pillar is lending against Friday. Could someone borrow the maximum on Sunday knowing Monday will be ugly? They could, at fifty percent loan to value, against a price that is at most a weekend old. That is the reason the loan to value is fifty and not eighty. The cushion between the maximum LTV and the liquidation threshold exists to absorb exactly this: the gap between the last price and the next one. We would rather lend less all week than lend more and hope the weekend is quiet.
The eighty hour figure is a parameter, not a law. As the feeds mature, it can come down. As the markets get more volatile, the LTVs can come down too. Both are single transactions by the protocol owner and both are events you can watch for. What will not change is the principle: when the oracle cannot see, the protocol does not guess.