if my dad said wear jeans and a t shirt - he would mean literally wear jeans and a t-shirt. I agree, though, that op knows her father best, but personally, i'd respect the wishes of my father at his birthday party and dress casual (casual for me would be nice jeans, a nice top, and wedge sandals).
I did talk to my mom shortly before we left and I guess that my dad told her that "what you're wearing now is fine." At the time, she had just come inside from gardening and was wearing her scrubby gardening t-shirt and shorts. Not exactly something she would ever wear for anything but gardening. And she did mention that apparently he did send an email out to everyone telling them it was casual.
But after looking at the weather forecast, I just couldn't fathom wearing jeans...it was going to be in the 90s and HUMID!! So, I decided on what I deemed a fairly casual cotton dress (and something I wear regularly on weekends) with flat gladiator sandals. Dad ended up complimenting the outfit and a lovely evening was had by all (and there was a wide range of outfits from all my relatives - dad ended up wearing cargo pants and a button down shirt, my mom wore a tunic and a jeans, there was another light dress, capris).