Absolutely the time and day can effect your spell casting. The days of the week are governed by specific planets. Sunday is governed by the Sun, Monday is the Moon, Tuesday is Mars, Wednesday is Mercury, Thursday is Jupiter, Friday is Venus and Saturday is Saturn.
The hours are also governed by the planets. The thing is astrological hours and our normal hours are not the same. To figure out how long each hour is start off with getting the time of sunrise and the time of sunset. Once you have these, find out how many minutes there are between sunrise and sunset. Divide this number by 12 and this will tell you how many minutes ae in a solar 'daytime hour'. Do the same thing for the time from sunset to sunrise the next day. This will give you the length of the hours for the solar 'nighttime hours'.
Once you have determined the length of the hours, you can determine what planet governs the hours. The first daytime hour of ever day is ruled by the planet of the day. For example, the first hour of Monday would be ruled by the Moon. The planets always go in the same order: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. So, in the case of a Monday, the fist hour would be the Moon, the second would be Saturn, the third is Jupiter, etc.
Once you know the day and hours, you need to determine what planet governs the work you are trying to do. In the example you gave of the love spell, that would be ruled by Venus, so that would need to be done on Friday during the hour of Venus.
Ideally, you want to do your work on the day and time appropriate to the work. If not, you can do it any day, but on the appropriate hour. The third choice would be on the correct day but any hour.
Hope this helps.