EDIT okay, never mind, after finishing day 5 it gave me both achievements.

I'm playing catch up on the plot and I just finished day 4, but I never received an achievement or the item for the achievement... are they only available the first 24 hours? :( When I clicked to leave the manor for day 4, it immediately threw me into today's plot stuff, is that why..? Or did I miss something?

For other new users to Subeta, to get the Achievement for any previous Day to show up, you need to revisit Skelly Manor. Completing the next plot step isn't necessary to get the previous one to show up, that's just what worked for . There may also be some cache delay in between your visit and it showing up.