“rich, whose blessed key, Can bring him to his sweet up-locked treasure”
The poet compares himself to a wealthy man who possesses a key to a locked strongbox but chooses not to open it every hour. 'Blessed' here means fortunate or favoured rather than holy; 'up-locked' is a compound meaning sealed or shut away under lock. The whole opening quatrain establishes the controlling conceit: rarity, not abundance, is what makes pleasure intense.
historical The sonnet form- TLN 719historical topical
“feasts so solemn and so rare, Since, seldom coming in that long year set, Like stones of worth they thinly placed are”
'Solemn' in Shakespeare's English meant formally observed, dignified, or ceremonious — not gloomy. The Church calendar fixed a small number of 'solemn feasts' (major holy days such as Christmas, Easter, and Whitsun) amid ordinary days, which made them stand out precisely because they were rare. The simile 'like stones of worth they thinly placed are' compares feast days to precious gems set sparsely in a piece of jewellery — spacing is what lets each stone flash.
“Blessed are you whose worthiness gives scope, Being had, to triumph; being lacked, to hope”
The closing couplet turns from the argument about rarity to a direct compliment: the beloved's worth is so great that possession of him is a triumph, and even his absence provides hope — not grief. 'Scope' means room or occasion; 'triumph' carries its Roman sense of a victory celebration (a glorious, public joy), not merely success. The chiastic balance — 'being had... being lacked' — makes absence and presence two modes of the same value.
historical The sonnet form