There's no good time to go back, only the time that rent and health insurance choose for you. Your life is on fire and the office is discussing lunch. Both are real; the whiplash is the hard part.
What to tell your manager
Less than you think. "My father died. I'm back, but I'll be at reduced capacity for a while, and some days will be worse than others." A good manager needs the shape of it, not the story. If there's an HR bereavement policy, ask a colleague to find out what it actually says - policies are usually better than people assume, and worse than they should be.
What to tell coworkers
You'll get the head-tilt and "how ARE you?" in the kitchen. Have a stock answer ready so the question can't ambush you: "Getting through. Thanks for asking." It's true, it's complete, and it ends the conversation without ending the friendship.
The one-line email instead
If you'd rather not do any of it live: "Before I'm back on Monday - my mum died last month. I'd rather not talk about it at work, but I didn't want you to hear it sideways. Business as usual is genuinely what helps." Send it to the three people who matter; the office grapevine will do the rest, for once usefully.
And when you catch yourself working well and forgetting for an hour - that's not disloyalty. That's an hour off. Take it.