Shares are delivered to the buyer's Demat account two trading days following the transaction date in India. If you're the buyer, your stockbroker will let you sell these shares before they arrive in your Demat account, as long as the exchange delivers them on schedule.

If the exchange does not deliver the shares to the buyer, it signifies the seller failed to deliver the shares to the exchange in this transaction. In stock market jargon, this failure to deliver shares is referred to as "short delivery." The exchange then organises an auction for the same number of shares and delivers them to the buyer.
You will be able to see the shares tagged as 'T1 holdings' on trading portal till T+2 day if you purchased shares on Monday (let's call this T day) (Wednesday). If the shares haven't been delivered to your account by Wednesday, you won't be able to see them in your trading portal holdings on T+3 day (Thursday).

You will be able to see the shares on trading portal from the next trading day once the stock exchange distributes the shares on the T+3 day (Thursday) (Friday).

Short delivery can occur in equities with little liquidity or when a short MIS/BO/CO hasn't been squared off in certain situations. You will be advised of this via SMS and email in this situation.
Note: In the case of physical settlement of derivatives, if the seller fails to deliver, the exchange holds an auction on T+3 days. (The settlement time will be T+4 days in this situation.)