pixel_player87635841 next time you see an free event pop up ad, click OK. Then verify you have a gift icon in your camp. It'll have a countdown timer to let you know how long you have left to accept the event. As was said previously, your time to complete the event starts the day you accept it.
🐦I should add, I play only on mobile and my gift icon has a timer. However, I just saw 2 screenshots in the following link, of both PC and mobile gift icons, PC doesn't seem to have it.