僕は今日は一生懸命働く様な気分じゃない。

。

I'm not in the mood to work hard today.

Literal

I [topic-は] today [contrast-は] diligently work [resemblance-ような] mood [copula-じゃ] not.

~ような気分じゃない ('not in the mood to'). 一生懸命 (いっしょうけんめい, 'with all one's might') is a very common adverb. 様な is a written variant of ような. 気分 ('mood'). Triple は stacking is notable — 僕は, 今日は, and the implicit contrast.