During season six or seven...but if you want to change it then ok.

A few days before Valentine's Day Spike over heard Dawn upset over the fact that no guy likes her and at school she's not going to get a single Valentine gift. She's upset she's going to look like a total dateless goober in front of everyone at school! 

Spike is not going to let his Bit be laughed at so he secretly has flowers,balloons and candy all sent too her. I was thinking Dawn knows it was sent by him and due to friendship but she lets all the girls at school crow in envy over it and lives off the glory.

The idea is that when Buffy gets wind of this....and the fact Spike didn't send her anything....she gets jealous and confronts him over it!