Provided that C is a cell array that contains only strings,
monthnames = {'Jan', 'Feb', 'Mar', 'April', 'May', 'June', 'Jul', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec'}; %or as appropriate
[tf, idx] = ismember(C, monthnames);
Now tf and idx will be arrays same size as C. tf(I,J) true indicates that C(I,J) matches one of the months, and for those locations, idx(I,J) tells you which of the months was matched.
If your cell array contains non-strings as well then
tf = cellfun(@(S) ischar(S) && ismember(S, monthnames));
and finding the index efficiently is trickier.
