From what I have learned the day that people met for worship is subjective. For the jewish (at least of the anciet times) God was believed to have rested on Saturday, so that was the sabbith. At sometime in the Catholic church history it was observed that Christ raised from the dead on a sunday, therefore mass was said on Sunday. My personal opnion is to worship that day that you can worship. Sunday is best for me. For some in the church I attend there's a Wednesday worship service. The day isn't as important as the action.
My experiences with tithing follows what you've said. When I give out of the love and faith of my heart then it follows as God said. When it's been an obligation, nothing. It's been a real learning experience for me!