Lol I don't agree with pumping up for a night out or a date, a guy used to come into the gym and he was in great shape, like a naturally genetically gifted arse because he tookroids, trained once a week (for 15mins to pump up on a Friday night) and ate pizza yet his delts were big his chest, his arms, a small waist with abs.
I think just go as you are, a bit of a pump won't make much difference and the date I take it has seen you before, not to mention most girls don't actually like muscles, strangely us men just like em lol.
Edit: Pulled a back muscle today, should of seen it coming. Just got out of bed and hadn't warmed up. Was just starting traps and felt the twang in my upper back. Hurts to lift arms over head and although its easing now, hurt to breath a it too.