Early September is the best time to scarify. This is the time when grass
produces side shoots and scarifying produces enough space for this growth to
take place.

Scarifying in spring damages new growth.

Scarifying can be undertaken from late spring throughout the summer to remove
moss that has been killed off by the use of lawn sand or a proprietary moss
killer.

Though some people may be able to get away with scarifying at various times of
the year the above are the generally accepted rules by those who look after
lawns on a professional basis (luxury/show lawns as opposed to utility/amenity
lawns).

If in doubt, leave well alone. Thatch is unlikely to do any real damage in the
next few months, or even the next year. The removal of moss and weeds, and the
occasional feed will do far more for your lawn than scarifying.

All the best,

Dave.