It's okay to have fun. In fact, it makes good business sense on monday and tuesday. The more you enjoy being around your colleagues, the better you'll be able to work together. Loyalty and relaxation are key themes at the start of the week, but wednesday and thursday will be about as far from relaxed as you can get. You'll be cutting conversations short simply because you don't have the time. Be polite about it and others will see your shortness as nothing more than an indication of how much you're getting done. Though you may be sitting at your desk on friday, your mind is elsewhere.