The answer to this can be found in Chapter 22.
First, you need to remember that the creature has told Victor that he will be with Victor on Victor's wedding night. Victor is now terrified that the creature will come back and murder him, possibly in front of Elizabeth.
Victor acts in two different ways because of this. First, he tries to act all happy so that no one will know that he is afraid. You can see this in the following quote:
...I felt my heart sink within me. But I concealed my feelings by an appearance of hilarity that brought smiles and joy to the countenance of my father, but hardly deceived the ever-watchful and nicer eye of Elizabeth.
But then Victor arms himself with a gun and knives. This makes him feel more secure, and so he starts acting a lot more like he normally would.
I carried pistols and a dagger constantly about me and was ever on the watch to prevent artifice, and by these means gained a greater degree of tranquillity.
As Victor's wedding night approaches, he begins to feel a sense of anxiety and despair, knowing that the monster will make good on his promise. However, Victor attempts to conceal his true emotions by acting happy in front of his father, even though he is not able to fool Elizabeth. Throughout the entire wedding preparations, Victor is able to maintain a joyful appearance. Simultaneously, Victor's anxiety gradually increases as his wedding approaches, and he begins to take precautions to protect himself from the monster. Victor arms himself by carrying pistols and daggers, which give him a false sense of security. Victor mentions that he is able to gain tranquility before his wedding because the threat begins to feel more like a delusion. However, Elizabeth's melancholy mood continually reminds him of the monster's promise, and the foreboding atmosphere surrounding his wedding returns.