Saturdays used to be your day. Nothing was said, no plans were made; it was just assumed that you'd be spending time together. But lately, it seems something more important to her comes up each week. Your weekly dates are no longer set in stone, and everything and everyone else takes precedent.
If you see your quality time slowly dwindling away, it absolutely points to her waning interest in the relationship, and it could mean she has transferred her affections elsewhere, or is at least entertaining the thought.
Independence is healthy, but a new-found focus on partying with her friends could mean she's putting herself out there again and is seriously considering pulling the plug on your love.