Theta — time decay — runs on the calendar, not on trading hours. Options lose time value every day toward expiration, and the weekend counts. So a contract you hold from Friday's close to Monday's open has quietly decayed across three calendar days while you couldn't do anything about it.
Why the weekend still costs you
Pricing models measure time to expiration in calendar days (adjusted for expected activity), because risk accrues continuously even when the exchange is closed — news breaks, gaps happen. The market partly “pre-pays” this by letting premium bleed ahead of the weekend, so you'll often see options soften into Friday afternoon and open Monday with the weekend's decay already taken out. Either way, the holder pays for time that passed with the market shut.
How much it matters
For a short-dated option, three days of decay is a meaningful slice of the premium — and you got no opportunity to react during it. For a 0/1-DTE scalper the point is usually moot, because those positions are closed the same day by design. It bites the trader who lets a “I'll give it the weekend” hope carry a losing Friday position into Monday.
Time value doesn't respect market hours. A weekend hold pays two days of rent for a position you can't even touch.
How to handle it
The disciplined default for short-dated options is flat by the close — you don't carry theta you can't manage. If you do hold intentionally (a longer-dated directional bet), price in the weekend decay before you enter, so Monday's lower open is expected, not a surprise. The habit of flattening at the bell is one of the reasons NoVo attaches an exit ladder to every position rather than leaving trades to hope.