I am of the opinion that when it comes to individual nutrients, there is no "this time of year" attached to them.
Calcium plays a role in the functioning of enzymes, is part of the structure of cell walls, helps control the water content of cells, and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium, once deposited in plant tissue, cannot move to other plant tissues, so must be supplied regularly.
Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. Many of the biological processes of the plants use Ca and Mg at the same time, so it's important to provide them together.
If you do a Google search for "MSU Fertilizer", "K-Lite" or "Cal-Mag", you are bound to find suppliers. Note that there are two "MSU" formulas - one intended for water supplies containing no minerals (the "RO" formula) and one for water supplies that do. The latter assumes that your water supply has enough "hardness" - usually consisting of calcium-, magnesium-, and iron carbonates - that supplementing the two elements is unnecessary.