"Great, date" and "weight": three words with wildly different spellings but the same vowel sound pronunciation! The words "date" and "weight" both follow common spelling patterns for the long a sound /eɪ/.
- Other 'a-consonant-e' examples: same, cake, safe
- Other 'e-i-g-h' examples: eight, weigh, sleigh
The word "great" is a non-phonetic word (it's spelling doesn't match it's pronunciation) and the long a pronunciation needs to be memorized. You can practice more long a words with audio.