I don't do theme days. Tried it, doesn't work when you're running a company where client situations change by the hour.
What actually works: I protect two deep work blocks every day. One in the morning before anyone can reach me - phone on airplane mode, no email, no Slack. That block is for strategic thinking, content, decisions that need real focus. The second is after lunch for operational work that needs concentration but isn't as creative.
The one schedule change that immediately reduced switching was brutally simple. I started checking internal messages only once per day. Not twice, not "just a quick peek" - once. Everything else waits or gets handled by my team.
Before that change my days were death by a thousand interruptions. I'd start working on something important, get pulled into a Slack thread, solve someones problem, try to refocus, get pulled again. By 5pm I'd been busy all day and accomplished nothing that actually moved the company forward.
When I restricted messages to once daily, two things happened. First, my team started solving problems themselves instead of defaulting to me. Second, I got back roughly 2 hours of uninterrupted focus that had previously been eaten by context switching.
Note this: The deeper lesson is that fragmentation isnt a scheduling problem - its a boundaries problem. Most founders don't need a better calendar system. They need the discipline to make themselves unreachable for meaningful stretches of time and trust that nothing will collapse while their offline.