Celsius does not kill or suppress annual bluegrass/poa annua. Please let us know what kind of turfgrass you have so that we can recommend a product that will work for your needs.
The seeding interval will vary depening on what type of turf you have. You should review the product label for specific directions for your turf. For example:
"Seeded Bermudagrass: This product may be applied to Bermudagrass up to 90 days prior to seeding without a significant reduction in Bermudagrass stand."
"Seeded Zoysiagrass: This product may be applied to zoysiagrass up to 90 days prior to seeding without a significant reduction in zoysiagrass stand."
Celsiu WG Herbicide is labeled to treat for Chamberbitter. However, we no longer carry Celsius and would recommend in its place Hi Yield Atrazine for ChamberBitter. Be sure to follow the product label to make sure your turf grass is tolerant of this product.
Per the Celcius WG Herbicide product label: MOWING INSTRUCTIONS: Do not mow immediately after treating with this product or before spray has dried.
Typically you should 1-2 days before and after applying.
Celsius will not remove bermuda from St Augustine grass as the label states it is safe for use on both turf types. If you are looking to remove bermuda from St Augustine grass there are very few products on the marke that will do that in general, much less on the most sensitive version of St Aug which is Floratam. Prograss EC Herbicide is one of the few listed specifically for removing bermuda from St Augustine lawns.
The best time to use Celsius WG Herbicide or any other herbicide will depend on where you are treating and the specific type(s) of weeds you need to treat. Most post-emergent herbicides will be most effective when the target weeds are young actively growing. If you need help with identification or determining the best time to apply, you can call your local cooperative extension office and ask to speak to the master gardener on staff and he/she should be able to help you.