Good times to hitchhike
Days of the week
Friday afternoon is definitely the best time for hitchhiking in most western countries. So many people are rushing to visit friends or family for the weekend. Also, everyone is in a great mood, leaving their work behind and looking forward for the weekend, which makes them more likely to pick you up.
Sunday morning until noon seems the worst time to hitchhike in Europe, and many other parts of the "developed" world influenced by Christianity (the US, Oz, Kiwiland and Canadialand come to mind). Cars are full of families, with a grumpy wife on the side and not too many working people on the road. It usually gets better in the afternoon, when people are driving back from their weekend trip alone. Sundays during extended weekends due to religious holidays (e.g. Easter) are even worse; make sure to start early enough and reach your destination way before the evening hours, or you'll be stuck by extremely low traffic, esp. along German highways.
On the other hand, on Sunday afternoon and early evening it is possible to catch fast & long lifts since many people visit their family, wife, husband boy/girlfriend who live far away during the weekend and have to return for work on Monday morning.
In Israel people work on Sundays so you'll be fine, but Shabbath is a horrible day for hitchhiking there since religious people are not driving at all (and they're usually quite skilled at picking up hitchhikers).
In many Muslim countries people also work on Sundays, and Friday is the day of the mosque, with considerably less traffic in countries like Jordan.
In a lot of European countries, most of the trucks are not allowed on the highways on Sundays until 22:00. They too are a hitchhiker-fond crowd. Sometimes. However, they all leaving their places at exactly 22:00. So, if you stuck late on a big service area, you can see them as a get-home-insurance. It's surely not a problem to find a truck to any destination if you start asking around at around 21:30.
Time of the year
In general, because of the length of the days and the cold Winter is bad, Autumn is also not that great, especially late Autumn. Spring and Summer are best, especially around the longest day, 21st of June (in the Northern hemisphere of course).