This week will be all about slippage: five days and five posts to understand and eliminate slippage.
Slippage isn't necessarily a problem. In fact, most clients expect it. Accidents happen, it's no one's fault - but hiding it is deliberate and unprofessional.
You must report slippage to your client as soon as it happens, while there is still time for them to find solutions and organise things on their end.
You are not working in a vacuum: your client will have made comitments based on your promise of delivery - if that is going to change you must let them know immediately and work with them to find out what you can do to support them.
If you want them to work with you, to collaborate, to meet you half-way when you have a problem, you must do the same thing to them.
It's not the slippage that's a problem as much as hiding it until the last minute.