What is theta / time decay, in plain English?
Theta is the rent you pay to hold an option. Every day that passes, the option loses a little value just because there is less time for it to work, even if the stock does not move at all. Theta measures how much slips away per day, and it is one of the most predictable forces in the whole game.
Picture an option as an ice cube you are holding for someone. Even if nothing happens, it is slowly melting. Theta is the melt rate: a theta of -0.05 means the option loses about 5 dollars per contract each day, all else equal.
And here is the deterministic part. Time decay is not linear. It speeds up as expiration approaches, and the last week or two melts fastest. That is exactly why buying short-dated options and then waiting is so brutal.
This is why sellers love theta and buyers fight it. If you are buying, you need the stock to move enough, and soon enough, to outrun the daily melt. If you are selling, that same melt is quietly working for you.
Weekends still count. Three days of decay can get priced into a Friday-to-Monday hold even though the market was closed. The clock does not care that you were away.