Especially if it’s before 6 am on a Saturday morning and your schedule for the day includes cleaning carpets, fortifying your domicile, food shopping and cooking dinner.
And especially if you are: tired, uninspired, suffering from the long-term effects of illness and would much rather be spending your time engaged in some mindless, emotionally satisfying pursuit, like looking at spaceship model kits online.
Of course following this advice is no guarantee that you won’t end up writing anyway, as ought to be evident by now.
Screw you, Bob Heinlein! You and your “always write” advice. What the hell did you know about blogs?