Last planting dates for spring
Decide what can still mature before the season changes.
The last safe planting day
Subtract the number of growing days from 28. Count forward from the morning you plant: a four-day parsnip planted and watered on Spring 24 can be picked on Spring 28. Planting it on Spring 25 misses the spring harvest. This assumes ordinary outdoor soil, no growth modifiers and daily watering.
What to buy on Spring 17
You have eleven growing nights left. A potato planted today can mature on the 23rd. A cauliflower needs twelve, so its normal harvest falls outside spring. If cash is tight, count the nights before spending it on the longer crop.
Mechanics reference: Crops · Stardew Valley Wiki.