Mayor and Council meeting agenda shows tonight's public hearing - on the question of whether the city should formally adopt a sanctuary city policy on illegal immigrants - will begin around 7:45 PM. I would suggest, particularly if you are speaking, to arrive between 6:00-6:30 PM. That is when the public portion of the Mayor and Council meeting is scheduled to begin (a closed session will be held at 6:00 PM, which is not open to the public).
There is a limit to how many people can fit in the Mayor and Council chambers at City Hall due to fire code, so there will likely be an overflow outside the room eventually. You would still be able to speak if you aren't in the room, but if you want a seat inside, I recommend you get there early.