Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Oh dear, this isn't a good habit for to be forming just now. In regards ***** ***** question, in situations like this, we do always need to tread with care. Especially since he is small (thus small diameter GI) and this certain could get stuck in the stomach (less serious) or lodge in the intestines (often when they need urgent surgery b/c it can affect circulation and the tissue there can die and require resection).
Ideally it should be removed ASAP. But if you cannot get Harvey seen, then you need to watch him closely. Any red flag signs and he must be seen by an ER vet (if his own vet isn't open). Red flags of trouble or obstruction include restlessness, lethargy, vomiting with blood or coffee ground type material, inability to keep any food or water down, anorexia, pale gums, belly pain, straining to pass feces or passage of black feces (digested blood).
If we don't have any of those signs, you can take a supportive/monitoring approach. We can feed small meals of a light diet (eg rice with boiled chicken, boiled white fish, scrambled egg, or meat baby food (garlic or onion free only) and add in fiber (eg spoonful of tinned pumpkin or all bran) to bulk up the stool and push this through the gut. Adding a GI lubricant (eg cat hairball medication Miralax, lactulose or food grade mineral oil) can help too. While doing this, again keep a close eye on him. And if those signs are seen he needs an check +/- xray to see where the sock is and if it needs to be removed by surgery (or if in the stomach, by scope).
Finally, just to note in case you were keen to have Harvye seen today, some vets in our country have office hours today. As well, I wanted to mention that most practices have contingency plans for emergency care for their patients even when they are closed. Therefore, it is worth ringing the practice. If they are open, you can get in today. If they aren't, then they will likely have a message to direct you on contacting their out of hours service. And if you don't have a vet, you can find a local one via the RCVS Register (http://findavet.rcvs.org.uk/find-a-vet/) or Vets Now (http://www.vets-now.com/find-an-emergency-vet/) who are open all nights/weekends. In any case, if you wanted to get this checked out sooner then there are options to do so.