I don't know whether the Lammas is still widely observed in English Christianity. Perhaps the deterioration of the climate in the northern hemisphere since circa 1200 A.D. has taken it out of phase with the agricultural work (though it remained listed on church calendars for many centuries). On the other hand, different varieties of grain may now be in use, with earlier harvest times than were used in the low middle ages. Sorting out these factors would be a fascinating interdisciplinary project.
The U.S. Episcopal Church calendar doesn't have Lammas day. Of course, the seasons aren't precisely the same in all parts of the U.S. as in England.