When should I scarify my lawn?
The answer is "It depends"!
Traditionally scarifying takes place in the spring and autumn.
In the UK that would be April / early May or late August / early September - the times when grass growth are at the maximum.
You can do it any time as long as you understand the results or the grass growth.
If you really want to clear that moss or thatch from your lawn then go ahead and do it anytime, but be aware that if you do it outside of these times it could leave your lawn in a mess or even kill the grass as well as the moss.
Clever gardeners will watch the weather and use the scarifier when they know there will be sufficient rain fall to encourage the grass to grow and repair the damage.
I was pretty certain we were due a huge downpour overnight so I whipped that scarifier out and did a bit of light scarifying (and removed a HUGE amount of moss).
In a couple of days I probably won't know the difference.