Something special have been occured in "今日". For example, someone's birthday or anniversary. Or speaker got very tired by something "今日". Because of that, the speaker wanted to say "Please forgive me at least today…" Example sentence 今日くらいは家族と過ごそう。(weblio辞書) Let's spend at least today as a family. くらい=ぐらい Alternative phrase ・今日だけは… ・せっかくの○○なので今日は…