It is the act of pampering oneself in a specific way. The usual three "S's" (shit, shower, shave) but with a few things added in. First is masturbating, preferably before entering the shower. While showering ensure you have a can of your favourite beer (if you are a risk taker feel free to have beverages in glass containers) and put on some music (ZZ Top comes to mind). Voilà, a pampered gentleman.
Friend: What did you do this weekend?
You: Well I stayed in on Sunday and had a Gentleman's Spa Day
Friend: Nice