Any day that you choose to visit a Tampa art gallery would be a good one. A big part of the reason what that is the case has to do with the fact that Tampa is a city with a really strong artistic culture to it, one that does not compromise on experimentation in any way, shape or form. Hence, you can probably get a good experience by just randomly visiting an art gallery at any given point in time, but you should also know that in certain situations you might be better off visiting when a special exhibit has been put on display.
These exhibits only come around once in a way, and this is part of the reason why they are so amazing. All of the art galleries to visit in Tampa are going to have special exhibits every now and again. They would most often involve the work of a really famous artist that has been working on a new exhibition and is going on tour to show this exhibit to as many people as possible. If you are lucky enough to find yourself in Tampa in time for their exhibit then you should treat yourself and go see what they have been working on.
This would also give you the chance to actually talk to the artist and see what they were trying to do with each piece. Gaining an in depth understanding of art galleries and the like is really important when you realize that the art is usually meant to be viewed in certain contexts that are not going to be all that obvious without input from the artist that actually made the piece and worked so hard on it.