I usually try to work on them when I'm inspired, but I have toddler now, so I'm finding it necessary to allocate specific blocks of time. Unfortunately, sometimes my brain doesn't seem to work, so that thin slice of allocated time doesn't allow for side project work at all times.

I feel you. Same boat here. I don't even have the mental energy to start and work on an actual "project". It's really difficult to learn new things after 930pm each day, and my guy can't nap alone either, so my only coding time during the week is when my brain is mush from a full day of toddler wrangling. I get some time for reading and synthesis during naps, and I try to tackle more bite-sized things like Advent of Code or working on my blog at night. On weekends, I usually get a chunk of time during the day, but like you mentioned, I'm not always in the mood to use it to good effect. 🎲 🎲

Just writing the above was effortful while my guy is asking over and over that I open the paint container and shaking it in my ear 😂

Well I know the feeling. I have a three year old here and I rarely have the energy to work on side projects in the afternoon after she went to bed. Now and then I can take an hour or so directly before or after work. And luckily my current employer has a 20% slack time rule for all employees that I can sometimes use for side projects even when they are not directly related to the companies interests.

Normally late at night. It's when I feel motivated, I won't feel like working on anything till 10-12 pm, but I have to try to get to bed around 11:30... It's frustrating.

For me, it's a bit of each: I will schedule blocks of time at weekends to work on OSS projects, but I'll also work on them when I feel like it on a weekday evening, and sometimes if I get a flash of inspiration I might work on them during the workday too during a short break from work (or full-on during work if its a library we depend on at work).

A time-management question asked on Slack might get answers skewed toward the more chaotic end of the spectrum.

When I have time I don't 😟