Harry had decided that during his wedding, he wanted to have a mother and son dance. He knew most people would think it'd be weird because he wasn't biologically related to Molly, but he wanted to show her his appreciation for everything she had done for him over the years.
When he asked Ginny about it, she said he would have to do it while she was having her father daughter dance, as she wouldn't want to bore the guests. She also said that he should surprise Molly and ask her spontaneously (her reasoning was that it would show his appreciation more genuinely). He agreed to doing it while his wife had her father daughter dance and decided that asking Molly on the spot would be the best way to go.
When the time came to ask her, he couldn't have been any more nervous. He had no idea why, though, as he had just married the love of his life. But when the music started playing, he quickly walked over to her and offered her his hand. "Will you dance with me?" he asked as he smiled at his mother-in-law.
"Of course," Molly said as she took Harry's hand, stood up and looked at Andromeda Tonks quizzically. Both of them busted out laughing, as if they were back at Hogwarts again.
"What's so funny?" Harry asked as he led Molly to the dance floor.
"Nothing, love," Molly said reassuringly. "I just wasn't expecting this, is all," she said as she followed Harry to the middle of the dancing floor.
Resting her hand on Harry's shoulder, Molly took a hold of Harry's hand and intertwined her fingers with his.
"What is this all about?" she asked when she felt Harry rest his free hand on her hip.
"I felt this was the best way to show my appreciation for everything you've done for me over the years," Harry replied as he started to dance.
"You know you're like another son to Arthur and I, now it's just official," Molly said as she looked up at Harry and genuinely smiled at him. She wasn't at all surprised by his answer.
"I know," Harry said as he felt Molly rest her head on his chest. "But still, you're the closest thing I've had to a mother," he added as he smiled down at his mother-in-law.
"Merlin's beard, Harry, you're going to make me cry," Molly choked out. She had already done enough crying for the evening and she didn't want to ruin this for Harry.
"That wasn't my intention, Mum," Harry said as he rested his head on top of Molly's head. He then felt a soft sob rack the petite body that belonged to his mother-in-law. He knew he had finally gotten his point across because that was the very first time he had ever called Molly 'mum' and it was no doubt, the last.