Anticipate when they will be at their grumpiest- That way you can plan things in when they are feeling their happiest and avoid doing things when all they want to do is mope around the house.
Relationships on Female First
Don't do things you know will tip them into a grump- If you know taking them to the shops, making them do DIY or asking them to do something they don't want to will induce a grump- go by yourself, as someone else or let them do the thing in their own time.
Let them have their mood- There's no point trying to suppress the grumps- let them ride it out and come around again. People can't snap out of a feeling they are so engrossed in so it's a waste of time and energy. They are entitled to be moody if they want to be.
Find other things to do- If you know when they will feel this way- go off and do your own thing. It will give them time and space let the emotion lose momentum and it stops them feeling like they're being watched or judged.
Don't punish them for feeling this way- You get grumpy too and they have to put up with it, so be fair to them and move on from it.
Don't make excuses- We are programed to provide excuses when we have extreme feelings to things. Your partner may not have a reason- they might just feel this way- so don't over examine what it is that has triggered it. Chances are it's nothing serious, just a passing mood.
Avoid telling them why they shouldn't be grumpy- Telling them that they have nothing to be grumpy about or making them appreciate everything they have won't work. Having a car, a house, a family, money, a job are not always prescriptions for happiness. You can have it all and still feel glum it's not ungrateful just human nature to not always be on top of the world.
Accept that they may need other people's company to bring them out of their slump- They love your dearly but a session with their friends or family might be the thing they need to help lift their mood. It's not that they are rejecting your help, but different people can be effective for different occasions.
Laugh it off- When they are in the moment- they probably won't see the funny side, but when they have escaped the feeling- they may initiate conversation about it in a jovial way. If they do- join in to show that you bear no grudge against them for having an off day.
Remind them it's a two way street- They may have been difficult to be around, feel bad about it and apologise. It's important to let them know that they have to take the brunt for your grumpy moods too. It's all about the rough with the smooth.