If one has decided to make aliya, are there particularly good (or bad) times during the year to do so, beyond the desire to be there for chagim? Factors that seem relevant would include tax-related timing/deadlines, the school year, employment cycles (when is it easier/harder to get jobs), rental cycles (if leases are easier to get at certain times of the year), availability of government services for olim, and probably other factors I haven't thought of.
From what I have seen there are 3 or 4 popular times to move to Israel.
-Right before or right after January 1st.
This is done mainly for tax purposes so that it is easier on your mind to remember when things are do for which country and you don't feel all confused. However this does not work well if you have children that need to enter the school system.
-A month or so before the Chagim (Rosh Hashana, Yom Kippur)
This is mostly done for religious reason as it just feels good. It is also good for children as the school year is just starting, and you can have a place for the children to go while you take care of all the things that need to be taken care of when you move.
-Right before Pesach
This is a time when Israel's weather is just about perfect, so moving heavy things or worrying about air conditioners and heaters is not so important. It aslo makes "cleaning for pesach" that much easier.
-The middle of the summer
Many families find that moving in the middle of the summer is best for the kids, as it seems like just one huge family vacation at first.
All Olim services and deadlines are based on the day you make Aliyah are not fixed to any yearly schedule. Except for specific laws that might be passed the year you make Aliyah, there is no real difference when you move. (For example, if they decide to change the import tax rates one year, that could be a factor... however that is very very rare.) As far as finding a Job, I have found that "hiring season" happens about once every 3-4 months depending on the industry.