It means her mood will pick up / get better. You can also use it on other situations- The sales have been low but will pick up around Christmas.

"pick up" in this case means to improve on what it was before. The clue is that up until that point the day had been awful. There are other meanings of the phrase, such as: "My usual pick up time is at 8am" - someone will come in a vehicle and collect me at that time. "Pick up that piece of paper!" - literally pick up some paper lying of the ground. "The train will soon pick up speed" - it will start slowly but get faster (this is like the day)

What a coincidence Today phrasal verb was (pick up ) It means many things Get sth. Collect Improve Increase

here the pick up refers positive thoughts means do not loose hope it can get better by the end of day.