Conventional wisdom is to send it as soon as possible after receiving your deferral notice (so December) unless you were/are waiting for a specific major result (e.g., being named one of the Regeneron STS scholars, perhaps publication in an academic journal, etc.) that you think would markedly improve the overall strength of your case.
Harvard will assume that all deferred candidates will still really want to attend, so it is implausible that stating so in a LOCI adds much to the quality or attractiveness of an applicant’s case. But, a LOCI is a low-effort exercise and one might as well send it in.
Good luck!