If you take into account how many calories you will be consuming on the day, YES!!
Losing weight really is as simple (but not simple at all) as burning more calories than you are consuming. Do this, you will lose weight. However, at the weekend you have a birthday brunch to go to, can you still go? If you account for this in the days leading up to the brunch day, you can absolutely still lose the weight you desire.
If you have a trainer/nutritionist to report to, you should be letting them know that you have a heavy weekend ahead so that they can plan out for you what is needed in order for you to be allowed to do this whilst staying on track with your goals. They can then work out exactly how many calories are needed to be consumed and burned to make way for the day of chaos!
It’s simple science, but it’s science that has to be precise in order for you to achieve your goals.
If you’re trainer is not doing this for you, they should be!
Attend that special occasion, just make sure that you earn the day off of the diet by working extra hard in the week leading up to it!