3 weeks on, one week off. And the week off has started with a "Wiped out Sunday" - so tired you can't get off the sofa, you snooze all afternoon and then wander around the house in a semi-daze. You're almost worried you are so tired.
You started the day relaxed but happy to not be doing any more training. As the day went on you've descended into weakness and super-tiredness. It's like you let go, relax and the long-term recovery starts to kick in. Your body no longer needs to hold itself ready for the next piece of exercise, instead it can start on the restoration project that it's been putting off whilst you've been hammering it.
And that's why you have a "Wiped out Sunday" - where you can totally get the rest. If you tried to start your recovery on a Monday it wouldn't be the same - going to work, starting the week, commuting - all that actually adds stress and delays restoration.
If you don't know what I mean about feeling wiped out then perhaps you aren't pushing yourself in your training? You should be going a little bit beyond yourself, doing a bit more than you can really cope with, and in the short term that means you'll be tired. In fact you'll have accumulated tiredness over the 3 weeks - a bit here, a bit there - you carry it whilst you are busy training and then it all gets to come out once you stop.
If this isn't happening for you then may be you need to have another look at your training, and at what you want to do this year?