The following's based purely off my own empirical data.
For any given synth, you have the option of facing 2-5 directions, and crafting on as many days, to noticably affect the results.
Facing THE CRYSTAL'S DIRECTION/on THE CRYSTAL'S DAY: Fail rate may decrease slightly. Fewer critical (lose all ingredients) fails.
Facing WEAKER CRYSTAL'S DIRECTION/on WEAKER CRYSTAL'S DAY: Seems to ct as "virtual skill +1, +2 for both"; the closer you get the gap to -4, the better skillups you get, however, if you close the gap further with this method skillups will become less common.
Facing STRONGER CRYSTAL'S DIRECTION/on STRONGER CRYSTAL'S DAY: The opposite of the previous. Worth facing this direction if you're close to the cap of an item and skillups have slowed down.
Facing LIGHT/on LIGHTSDAY: Acts as weaker to all but light and dark, but with less effect. Acts as stronger to dark, with full effect. Acts as same to light.
Facing DARK/on DARKSDAY: Acts as stronger to all but light and dark, but with less effect. Acts as stronger to light, with full effect. Acts as same to dark.
For HQs: This appears to be a seperate roll, with all day/facing modifiers reversed---those that reduce your skill increase HQ odds, and vice-versa.