Every bitcoin transaction that's sent flows into what's called the mempool (short for memory pool) before it can be confirmed by miners. When there's a dramatic spike in transaction activity, the mempool can become congested because so many transactions are waiting to be included in the next block.
Bitcoin users across the network may notice their transactions sit as unconfirmed or pending for a longer period of time, and we understand this can cause users to be concerned about the status of their funds.
In most cases, your transactions will eventually confirm. It may just take longer than usual to do so.
Note: The Blockchain Wallet has an Advanced Send option that allows users to specify a custom fee and this can come in handy to help ensure your transaction confirms quickly during periods of congestion. We recommend this option to experienced users with an understanding of transaction fees only.