Serious and without any humour. (Cambridge Dictionary)
Marked by the invocation of a religious sanction. (Merriam – Webster)
Marked by the observance of established form or ceremony. (Merriam – Webster)
Formal and dignified. (Oxford Dictionaries)
Not cheerful or smiling; serious. (Oxford Dictionaries)
Characterised by deep sincerity. (Oxford Dictionaries)
Someone or something that is solemn is very serious rather than cheerful or humorous. (Collins Dictionary)
‘We are united in grief’: Solemn Obama addresses Orlando massacre.
A Solemn Promise: Archbishop Aquila launches comprehensive webpage to speak to the people of northern Colorado.