With the caveat that it does depend on your job, as this isn't universal, for most PT workers, it makes sense not to work on a Monday because most BHs fall on a Monday if you are in England (4 always do, 1 is always a Friday and 3 move days).
With many contracts, this means you can't work because the business is closed, but have to use 7.5 hours leave, when you only get 3/5 of that amount, so you will be giving your employer hours each time. If you don't work Mondays, you gain hours from them.
Some people like to split the week, so they are never 'unavailable' to clients / service users / patients / customers / colleagues for longer periods of time.
Others (most of us I think) prefer to have a block of work then a block of time at home.
Personally, I wouldn't work 2.5 days, unless you can work 5 days a fortnight.
By the time you've got out the door in the morning, and commuted, then finished and commuted back, that supposed 1/2 day you thought you had with little one is mostly gone and fairly useless.
3 days is a really nice mix to keep your career but also have time at home with your little one.
Mondays and Fridays do tend to be the most popular non working days, so some like having different ones as quieter to go places or make appts, but others like the idea of meeting up with friends so prefer the more popular days.
Some work places might have busier days or more relaxed Fridays - again, some like to be in for those some like to avoid.
Really useful to be in for the day(s) your Team might commonly have Team meeting or training, or you find yourself really out of the loop. Only you will know if it is important to be in on particular days with your specific job.