Get back on the horse!
Coming off a holiday weekend is a perfect time to address this topic. I understand you are tired, tough shit. Working out a little bit tired this week is the bill you have to pay for the weekend of partying.
A lot of people do not have a problem starting a healthy lifestyle with clean eating and a regimented workout schedule when things go according to a routine on regular weeks. What people do have a problem with is staying on track once something derails them. During the summer we have a few obvious derailleurs with a 3-4 day Memorial Weekend, Fourth of July and Labor weekend at minimum that can trip up even the most devoted trainees. Throw in a wedding or two over a summer and the summer can turn into constant distraction from progress.
People come back from these weekends exhausted. They eat too much, they drink too much and they sleep too little. Do this several times over a summer and it becomes a perfect formula for fat gain. Fat that many people worked really hard to burn off from January to Memorial Day.
This is also why the Thanksgiving through Christmas time is such a problem for people. People will go completely off their diet and workout routine over Thanksgiving, then with Christmas shopping and other holiday season commitments they don't get back on track until after New Years when they have added 10+ pounds. There is no reason why the Thanksgiving weekend has to turn into 6 weeks of sloppy eating and minimal exercise. But the reason why this happens is so simple, people get off track then start making excuses.
How to Get Back
Go to the gym, tonight! If you workout from home, then do that after work tonight! It doesn't even matter if you normally follow a strict program, use this week to just do fun stuff you like at the gym. Get back on a regular Strength, Conditioning or whatever your goal is next week. We already lost Monday of this week for anyone extending their weekend that extra day. Get moving today, break a sweat, do circuits. JUST DO SOMETHING!!
It doesn't even matter what you actually do when getting back on track. Do something that makes you sweat, do it a few more times through this week, then hit next Monday ready to get back onto a more regular routine with body splits, power vs reps, whatever.
Do not let aHoliday weekend set you back weeks and months.