Rules for naming a startup
1. Easy to pronounce 2. Easy to spell 3. Easy to remember 4. As few syllables as possible 5. .com unless very good reason otherwise 6. Related to the product/service
Calen.com would be a no go if it were me because it violates rules 1,2,3 and 6.
I’d go with something like habitkeeper.com which is only $2.5k.
I would add uniqueness as a good criteria because it helps with SEO and branding and because unique names are often easier-to-remember (#3) than generic names.
What I would recommend if you want to go the calendar-based route is start with "cal", "calen", or "calend" and find a random syllable to complete the word in a unique and phonetically-obvious way. You probably can find something suitable for under $10k.
Or if you don't mind spending some money, calenda is 30k