You could try searching tags, but not everyone uses a SEP-assigned tag when they post. You could also do a general keyword search within titles - but I have to warn you that #1 not everyone uses the same terminology, and #2, the number of people who post about a specific SEP day drops off sharply after the first few days. After that, they just post about whatever topic they want to address.
Like TMS, and like life, it's messy. Embrace the messiness (aka: let go of perfectionism) .