Can you guess the search phrase?

The search term is likely to be "Flowers".

Flowers are sent in the highest volumes on Valentines day and Mother's day.

Both graphs have spikes in February. In Britian, Mother's day is in March. In the US it is in May. These correspond directly to the spikes presented.

Looks like something to do with taxes, but I can't figure out the phrase that matches his trends. "Taxes", "income tax" and "tax forms" all produce trends that have similar peaks as his, but nothing has that distinctive rise and dip at the end of the year.

Neat idea. This seems like the sort of thing that could be done with genetic algorithms - the problem has a natural objective criteria and evolving a search string is relatively straightforward.

First guess was football, since it would be seasonally periodic, and would spike in different times of the year due to the different international seasons... but the peaks aren't in the right spot, so no dice =(

I was going to say "taxes" ....