It states (32:5) מָחָר 'לה חַג וַיֹּאמַר אַה ֲרֹן וַיִּקְרָא, "Aharon called out and said, 'A holiday for Hashem tomorrow!'" If we were to read this pasuk without Rashi's commentary, it would seem that Aharon was encouraging the nation to make a holiday for the egel they just created. However, Rashi writes, "Aharon's intention was for heaven. He was certain that Moshe would come and they would serve Hashem."
This is an indication that a person can turn around in a moment. Today, they worship avodah zarah; tomorrow, they can do teshuvah and serve Hashem.
When a person turns around and faces the other direction, everything that was on his left side until now is on his right side. This tells us that with one turn, one improvement, one can turn around from being on the left to being on the right. This is the power of teshuvah; it changes the entire reality and situation.
We also find in Megilas Esther that changes can happen in a moment. Haman was at the highest level; the king's servants were all bowing to him, he wore the king's ring on his finger, and Mordechai HaTzaddik was at a very low level, and there was a tall tree prepared to hang him on. But in one moment, הוא ונהפוך, everything turned around. The king chose to hang Haman on the tree that was prepared for Mordechai, and Mordechai was led through the streets of Shushan wearing the king's clothing. This tells us that things can turn around in a moment. If a person sets himself to do Hashem's service, he will also turn around. He will reach the highest levels.
This can be alluded to in the Gemara (Megillah 7b) says, ידע דלא עד בפוריא לבסומי איניש חייב מרדכי לברוך המן ארור בין, "A person must intoxicate himself on Purim until he doesn't know the difference between the curse of Haman and the brachah of Mordechai." We can explain that even a person who is a rasha like Haman can, in a moment, become great like Mordechai HaTzaddik. It is possible to do teshuvah in a minute. This is the meaning of ארור בין ידע דלא עד ...איניש חייב מרדכי לברוך המן. A person is obligated to be aware that he can go from being like Haman to becoming like Mordechai HaTzaddik without there being a time gap.
