That's a little like asking 'how do I use WeeWX?'.
SummaryByDay and
SummaryByYear are provided to facilitate summary reports covering each day and each year respectively. They are used by creating a suitable report template using whatever tags you want (
iteration is frequently used) and then including the template under a
[[SummaryByXXXX]] stanza under
[CheetahGenerator] in
skin.conf. The best way to learn about how they are used is by having a look at the NOAA format templates and their usage in the Standard and Seasons skins.
Maybe if you could give us some more detail about exactly what it is you are trying to do you might get some more specific help.